Today, I have a few interviews lined up. I want to do well of course, so I thought I would lay out some interview review.
Let's begin!
A really awesome review is linked here- http://www.sanjayahuja.com/Interview%20questions.pdf
--------------
This information came from- http://careerride.com/embedded-systems-interview-questions.aspx
Example:
While(1) {}
OR
for(;;);
{
//Code
}
Embedded systems need infinite loops for repeatedly processing/monitoring the state of the program. One example could be the case of a program state continuously being checked for any exceptional errors that might just occur during run time such as memory outage or divide by zero etc.,
For e.g. Customer care Telephone systems where in a per-recorded audio file is played in case the dialer is put on hold..
Also circuits being responsible for indicating that a particular component is active/alive during its operation by means of LED's.
Memory allocation is another aspect that is optimized and it also makes sense to group a set of functions related in some way as one single unit rather than having them to be dispersed in the whole program.
In case of interactive systems display of menu list and reading in the choices of user's could be encapsulated as a single unit.
Here the use of registers is optimized as most of the memory access operations are limited to store and load operations.
Fewer and simple addressing modes, and simple instruction formats leads to greater efficiency, optimization of compilers, re-organisation of code for better throughput in terms of space and time complexities. All these features make it the choice of architecture in majority of the Embedded systems.
CISC again have their own advantages and they are preferred whenever the performance and compiler simplification are the issues to be taken care of.
However, the RISC strategy also brings some very important
advantages. Because each instruction requires only one clock cycle to
execute, the entire program will execute in approximately the same amount
of time as the multi-cycle "MULT" command. These RISC "reduced
instructions" require less transistors of hardware space than the complex
instructions, leaving more room
The Performance Equation
The following equation is commonly used for expressing a computer's performance ability:
The CISC approach attempts to minimize the number of instructions
per program, sacrificing the number of cycles per instruction. RISC does
the opposite, reducing the cycles per instruction at the cost of the number
of instructions per program.
Physical device drivers can’t perform all the logical operations needed in a system in cases like IPC, Signals and so on...
The main reason for having virtual device drivers is to mimic the behaviour of certain hardware devices without it actually being present and these could be attributed to the high cost of the devices or the unavailability of such devices.
These basically create an illusion for the users as if they are using the actual hardware and enable them to carryout their simulation results.
Examples could be the use of virtual drivers in case of Network simulators,also the support of virtual device drivers in case a user runs an additional OS in a virtual box kind of a software.
DMA modules usually take the control over from the processor and perform the memory operations and this is mainly because to counteract the mismatch in the processing speeds of I/O units and the procesor.This is comparatively faster.
It is an important part of any embedded systems,and the reason for their use is that they can be used for bursty data transfers instead of single byte approaches.
It has to wait for the systems resources such as the system bus in case it is already in control of it.
When we consider a several bytes of data say for instance 4 bytes of data,XYZQ the lower byte if stored in a Higher address and others in successively decreasing addresses ,then it refers to the Big Endian and the vice versa of this refers to Little Endian architecture.
Intel 80x86 usually follows Little Endian and others like IBM systems follow Big Endian formats.
If the data is being transmitted care has to be taken so as to know as to which byte,whether the higher or the lower byte is being transmitted.
Hence a common format prior to communication has to be agreed upon to avoid wrong interpretation/calculations.
Usually layer modules are written so as to automate these conversion in Operating systems.
"Little Endian" means that the low-order byte of the number is stored in memory at the lowest address, and the high-order byte at the highest address. (The little end comes first.) For example, a 4 byte LongInt
^Found from http://people.cs.umass.edu/~verts/cs32/endian.html
We normally use the #define directive in case we need to define the values of some constants so in case a change is needed only the value can be changed and is reflected throughout.
#define mul(a,b) (a*b)
The major disadvantage of macros is that they are not really functions and the usual error checking and stepping through of the code does not occur.
Inline functions are expanded whenever it is invoked rather than the control going to the place where the function is defined and avoids all the activities such as saving the return address when a jump is performed. Saves time in case of short codes.
inline float add(float a,float b)
{
return a+b;
}
Inline is just a request to the compiler and it is upto to the compiler whether to substitute the code at the place of invocation or perform a jump based on its performance algorithms.
- PC being overheated.
- Dust having being accumulated all around.
- CPU fans not working properly .
- Faulty power connections.
- Faulty circuit board from where the power is being drawn.
- Support Drivers not having being installed.
Debugging steps which can be taken are:
- Cleaning the system thoroughly and maintaining it in a dust-free environment.
Environment that is cool enough and facilitates for easy passage of air should be ideal enough.
- By locating the appropriate support drivers for the system in consideration and having them installed.
External signals,some condition in the program or by the occurrence of some event,these could be the reasons for generation of an interrupt.
Interrupts can also be masked so as to ignore them even if an event occurs for which a routine has to be executed.
Following steps could be followed to reduce the latency
- isrs being simple and short.
- Interrupts being serviced immediately
- Avoiding those instructions that increase the latency period.
- Also by prioritizing interrupts over threads.
- Avoiding use of inappropriate APIs.
Fork is the system call that is used to create a child process. It takes no arguments and returns a value of type pid_t.
If the function succeeds it returns the pid of the child process created to its parent and child receives a zero value indicating its successful creation.
On failure, a -1 will be returned in the parent's context, no child process will be created, and errno will be set
The child process normally performs all its operations in its parents context but each process independently of one another and also inherits some of the important attributes from it such as UID, current directory, root directory and so on.
One application where it is most widely used is when the mobile phone hangs and no activity takes place,in those cases watchdog timer performs a restart of the system and comes to the rescue of the users.
It is used to reset to the original state whenever some inappropriate events take place such as too many commands being given at the same time or other activities that result in malfunctioning of the GUI.It is usually operated by counter devices.
-Documentation providing for the minimum, average, and maximum number of clock cycles required by each system call
-Interrupt response times should be very minute.
-Context switch time should be very low.
-Compatibility with several plugin devices.
- Overall it should be very reliable.
Features like exception handling,simple syntax and Automatic Garbage collection all work in its favour as the language for use in ES's.
Also that it is widely used in the form of Java applets makes it very popular confining it to the limits of JVM.It is Dynamic in nature.
Its use is also being exploited in enterprise systems in the form of J2EE ,J2SE
J2ME in case of mobile applications.
- A semaphore S is basically an integer variable that apart from initialisation is accesses only through atomic operations such as wait() and signal().
- Semaphore object basically acts as a counter to monitor the number of threads accessing a resource.
- Mutex is also a tool that is used to provide deadlock free mutual exclusion.It protects access to every critical data item.if the data is locked and is in use,it either waits for the thread to finish or awakened to release the lock from its inactive state.
- Malfunctioning of address lines due to a short in the circuit
- Malfunctioning of Data lines.
- Some memory locations being inaccessible in storage due to garbage or errors.
- Improper insertion of Memory devices into the memory slots
- Faulty control signals.
In case of networking there may be several requests to access the same resource and all these are queued in a buffer and serviced in the order they are received
Hence to avoid the input/output units from getting overloaded with requests, we use multibyte buffers.
--------------
The following are basic interview questions for embedded systems - found http://careerride.com/embedded-systems-interview-for-freshers.aspx
• It lists the opcodes ,addresses and errors detected by the assembler.
• List file is produced only when indicated by the user.
• It can be accessed by an editor and displayedon monitor screen or printed.
• Progammer uses this file to find the syntax errors and later fix them.
• A program is always executed byte by byte.
• Firstly,1st opcode 7D is fetched from location 0000 and then the value 25 is fetched from 0001 .
• 25 is then placed in the register R5 and program counter is incremented to point 0002.
• On execution of opcode 7F, value 34 is copied to register R7.
• Then addition of contents of R5 and accumulater takes place.
• Here all the opcodes are 8 bit forming a byte.
• It is used to define the 8 bit data in binary ,hexadecimal or decimal formats.
• It is the only directive that can be used to define ASCII strings larger than two characters.
• DB is also used to allocate memory in byte sized chunks.
• The assembler always converts the numbers lnto hexadecimal.
• It associates a constant value with data label .
• Whenever the label appears in the program ,constant value is subsituted for label.
• Advantage: The constant value occuring at various positions in a program can be changed at once using this directive.
• Syntax: label EQU constant value
• 1st letter should always be an alphabetic letter.
• It can also use digits and special characters ?,.,@,_,$.
• Label should not be one of the reserved words in assembly language.
• These labels make the progam much easier to read and maintain.
• The two unused bits are user defineable flags.
• Carry ,auxillary carry ,parity and overflow flags are the conditional flags used in it.
• PSW.1 is a user definable bit and PSW.5 can be used as general purpose bit.
• Rest all flags indicate some or the other condition of an arithematic operation.
• This flag is also called as the overflow flag.
• Here the output of the signed number operation is too large to be accomodated in 7 bits.
• For signed numbers the MSB is used to indicate the whether the number is positive or negative.
• It is only used to detect errors in signed number operations.
Which register bank is used if we use the following instructions
• Statement A sets 3rd bit of flag register.
• Statement B sets 4th bit of flag register.
• Therefore register bank 3 is initiated .
• It uses memory location 18H to 1FH.
• The register bank is also called as R3.
• Stack pointer is incremented or decremented according to the push or pop instruction.
• If the stack pointer is decremented it uses locations 7,6,5… which belongs to register bank 0.
• If a given program uses R1 then stack is provided new memory location.
• The push instruction may also take stack to location 0 i.e.it will run out of space.
• It is called as jump if no carry( conditional jump instruction).
• Here the carry flag bit in PSW register is used to make decision.
• The processor looks at the carry flag to see if it is raised or not.
• If carry flag is 0 ,CPU fetches instructions from the address of the label.
MOV A,#55H
AGAIN: MOV P1,A
ACALL DELAY
CPL A
SJMP AGAIN
DELAY:
MOV R5,#9
HERE1: MOV R4,#242
HERE2: MOV R3,#255
HERE3: DJNZ R3,HERE3
DJNZ R4,HERE2
DJNZ R5,HERE1
RET
• Here the delay produced is 9*255*4MC*90=199,940 micro seconds.
• CPL is used to toggle the bits of P1.
• Short jump is jusd to produce a continuous loop.
• Port 0 is an open drain unlike ports 2,3,4.
• To use it as input or output the 10k ohm pull-up resisters are connected to it externally.
• To make port 0 as input port it must be programmed by writing 1 to all bits.
• Example:
MOV A,#0FFH
MOV P0,A
• Port0 uses pins 32 to 39 of 8051 to give the lower address bits(AD0-AD7)
• Port2 uses pins 21 to 28 of 8051 to give the higher address bits(A8-A15)
• This 16 bit address is used to access external memory if attached.
• When connected to external memory they cannot be used as input output ports.
• Here only single bit is accessed and rest are unaltered.
• SYNTAX: “SETB X. Y”.
• Here X is the port number and y is the desired bit.
• Example: SETB P1.2
Here the second bit of port 1 is set to 1.
• CPL bit : complement the bit (bit= NOT bit).
• JB bit,target: Jump to target if bit equal to 1.
• JNB bit,target: Jump to target if bit is equal to 0.
• JCB bit,target: Jump to target if bit is equal to 1 and then clear bit.
Let's begin!
A really awesome review is linked here- http://www.sanjayahuja.com/Interview%20questions.pdf
--------------
This information came from- http://careerride.com/embedded-systems-interview-questions.aspx
What is the need for an infinite loop in Embedded systems?
Infinite Loops are those program constructs where in there is no break statement so as to get out of the loop, it just keeps looping over the statements within the block defined.Example:
While(1) {}
OR
for(;;);
{
//Code
}
Embedded systems need infinite loops for repeatedly processing/monitoring the state of the program. One example could be the case of a program state continuously being checked for any exceptional errors that might just occur during run time such as memory outage or divide by zero etc.,
For e.g. Customer care Telephone systems where in a per-recorded audio file is played in case the dialer is put on hold..
Also circuits being responsible for indicating that a particular component is active/alive during its operation by means of LED's.
How does combination of functions reduce memory requirements in embedded systems?
The amount of code that has to be dealt with is reduced thus easing the overhead and redundancy is eliminated in case if there is anything common among the functions.Memory allocation is another aspect that is optimized and it also makes sense to group a set of functions related in some way as one single unit rather than having them to be dispersed in the whole program.
In case of interactive systems display of menu list and reading in the choices of user's could be encapsulated as a single unit.
A vast majority of High Performance Embedded systems today use RISC architecture why?
According to the instruction sets used, computers are normally classified into RISC and CISC. RISC stands for 'Reduced Instruction Set Computing' .The design philosophy of RISC architecture is such that only one instruction is performed on each machine cycle thus taking very less time and speeding up when compared to their CISC counterparts.Here the use of registers is optimized as most of the memory access operations are limited to store and load operations.
Fewer and simple addressing modes, and simple instruction formats leads to greater efficiency, optimization of compilers, re-organisation of code for better throughput in terms of space and time complexities. All these features make it the choice of architecture in majority of the Embedded systems.
CISC again have their own advantages and they are preferred whenever the performance and compiler simplification are the issues to be taken care of.
| CISC | RISC | ||
| Emphasis on hardware | Emphasis on software | ||
| Includes multi-clock complex instructions |
Single-clock, reduced instruction only |
||
| Memory-to-memory: "LOAD" and "STORE" incorporated in instructions |
Register to register: "LOAD" and "STORE" are independent instructions |
||
| Small code sizes, high cycles per second |
Low cycles per second, large code sizes |
||
| Transistors used for storing complex instructions |
Spends more transistors on memory registers |
The Performance Equation
The following equation is commonly used for expressing a computer's performance ability:
Why do we need virtual device drivers when we have physical device drivers?
Device drivers are basically a set of modules/routines so as to handle a device for which a direct way of communication is not possible through the user's application program and these can be thought of as an interface thus keeping the system small providing for minimalistic of additions of code, if any.Physical device drivers can’t perform all the logical operations needed in a system in cases like IPC, Signals and so on...
The main reason for having virtual device drivers is to mimic the behaviour of certain hardware devices without it actually being present and these could be attributed to the high cost of the devices or the unavailability of such devices.
These basically create an illusion for the users as if they are using the actual hardware and enable them to carryout their simulation results.
Examples could be the use of virtual drivers in case of Network simulators,also the support of virtual device drivers in case a user runs an additional OS in a virtual box kind of a software.
What is the need for DMAC in ES?
Direct memory access is mainly used to overcome the disadvantages of interrupt and progam controlled I/O.DMA modules usually take the control over from the processor and perform the memory operations and this is mainly because to counteract the mismatch in the processing speeds of I/O units and the procesor.This is comparatively faster.
It is an important part of any embedded systems,and the reason for their use is that they can be used for bursty data transfers instead of single byte approaches.
It has to wait for the systems resources such as the system bus in case it is already in control of it.
What is Endianness of a system and how do different systems communicate with each other?
Endianness basically refers to the ordering of the bytes within words or larger bytes of data treated as a single entity.When we consider a several bytes of data say for instance 4 bytes of data,XYZQ the lower byte if stored in a Higher address and others in successively decreasing addresses ,then it refers to the Big Endian and the vice versa of this refers to Little Endian architecture.
Intel 80x86 usually follows Little Endian and others like IBM systems follow Big Endian formats.
If the data is being transmitted care has to be taken so as to know as to which byte,whether the higher or the lower byte is being transmitted.
Hence a common format prior to communication has to be agreed upon to avoid wrong interpretation/calculations.
Usually layer modules are written so as to automate these conversion in Operating systems.
"Little Endian" means that the low-order byte of the number is stored in memory at the lowest address, and the high-order byte at the highest address. (The little end comes first.) For example, a 4 byte LongInt
Byte3 Byte2 Byte1 Byte0will be arranged in memory as follows:
Base Address+0 Byte0
Base Address+1 Byte1
Base Address+2 Byte2
Base Address+3 Byte3
Intel processors (those used in PC's) use "Little Endian" byte order.
"Big Endian" means that the high-order byte of the number is
stored in memory at the lowest address, and the low-order
byte at the highest address. (The big end comes first.)
Our LongInt, would then be
stored as:
Base Address+0 Byte3
Base Address+1 Byte2
Base Address+2 Byte1
Base Address+3 Byte0
Motorola processors (those used in Mac's) use "Big Endian" byte order.^Found from http://people.cs.umass.edu/~verts/cs32/endian.html
How are macros different from inline functions?
Macros are normally used whenever a set of instructions/tasks have to be repeatedly performed. They are small programs to carryout some predefined actions.We normally use the #define directive in case we need to define the values of some constants so in case a change is needed only the value can be changed and is reflected throughout.
#define mul(a,b) (a*b)
The major disadvantage of macros is that they are not really functions and the usual error checking and stepping through of the code does not occur.
Inline functions are expanded whenever it is invoked rather than the control going to the place where the function is defined and avoids all the activities such as saving the return address when a jump is performed. Saves time in case of short codes.
inline float add(float a,float b)
{
return a+b;
}
Inline is just a request to the compiler and it is upto to the compiler whether to substitute the code at the place of invocation or perform a jump based on its performance algorithms.
What could be the reasons for a System to have gone blank and how would you Debug it?
Possible reasons could be,- PC being overheated.
- Dust having being accumulated all around.
- CPU fans not working properly .
- Faulty power connections.
- Faulty circuit board from where the power is being drawn.
- Support Drivers not having being installed.
Debugging steps which can be taken are:
- Cleaning the system thoroughly and maintaining it in a dust-free environment.
Environment that is cool enough and facilitates for easy passage of air should be ideal enough.
- By locating the appropriate support drivers for the system in consideration and having them installed.
Explain interrupt latency and how can we decrease it?
Interrupt latency basically refers to the time span an interrupt is generated and it being serviced by an appropriate routine defined.,usually the interrupt handler.External signals,some condition in the program or by the occurrence of some event,these could be the reasons for generation of an interrupt.
Interrupts can also be masked so as to ignore them even if an event occurs for which a routine has to be executed.
Following steps could be followed to reduce the latency
- isrs being simple and short.
- Interrupts being serviced immediately
- Avoiding those instructions that increase the latency period.
- Also by prioritizing interrupts over threads.
- Avoiding use of inappropriate APIs.
How to create a child process in linux?
Prototype of the function used to create a child process is pid_t fork(void);Fork is the system call that is used to create a child process. It takes no arguments and returns a value of type pid_t.
If the function succeeds it returns the pid of the child process created to its parent and child receives a zero value indicating its successful creation.
On failure, a -1 will be returned in the parent's context, no child process will be created, and errno will be set
The child process normally performs all its operations in its parents context but each process independently of one another and also inherits some of the important attributes from it such as UID, current directory, root directory and so on.
Significance of watchdog timer in Embedded Systems
Watchdog timer is basically a timing device that is set for predefined time interval and some event should occur during that time interval else the device generates a time out signal.One application where it is most widely used is when the mobile phone hangs and no activity takes place,in those cases watchdog timer performs a restart of the system and comes to the rescue of the users.
It is used to reset to the original state whenever some inappropriate events take place such as too many commands being given at the same time or other activities that result in malfunctioning of the GUI.It is usually operated by counter devices.
If you buy some RTOS, what are the features you look for in ?
-Deterministic operating system having guaranteed worst-case interrupt latency and context-switch times.-Documentation providing for the minimum, average, and maximum number of clock cycles required by each system call
-Interrupt response times should be very minute.
-Context switch time should be very low.
-Compatibility with several plugin devices.
- Overall it should be very reliable.
Why is java mostly used in embedded systems?
Java was mainly designed and conceputalised for code that can work on different platforms without any hassles and also for being secure enough so as to not harm or corrupt other modules of code.Features like exception handling,simple syntax and Automatic Garbage collection all work in its favour as the language for use in ES's.
Also that it is widely used in the form of Java applets makes it very popular confining it to the limits of JVM.It is Dynamic in nature.
Its use is also being exploited in enterprise systems in the form of J2EE ,J2SE
J2ME in case of mobile applications.
Differentiate between mutexes vs semaphores
-Semaphores is a synchronisation tool to overcome the critical section problem.- A semaphore S is basically an integer variable that apart from initialisation is accesses only through atomic operations such as wait() and signal().
- Semaphore object basically acts as a counter to monitor the number of threads accessing a resource.
- Mutex is also a tool that is used to provide deadlock free mutual exclusion.It protects access to every critical data item.if the data is locked and is in use,it either waits for the thread to finish or awakened to release the lock from its inactive state.
What are the commonly found errors in Embedded Systems?
- Damage of memory devices due to transient current and static discharges.- Malfunctioning of address lines due to a short in the circuit
- Malfunctioning of Data lines.
- Some memory locations being inaccessible in storage due to garbage or errors.
- Improper insertion of Memory devices into the memory slots
- Faulty control signals.
What is the need for having multibyte data input and output buffers in case of device ports?
It’s normally the case that some devices transfer the output either in a bursty or a sequential manner and also during input entry. If we take the example of keyboards, all the data entered is stored in a buffer and given at a time or one character at a time.In case of networking there may be several requests to access the same resource and all these are queued in a buffer and serviced in the order they are received
Hence to avoid the input/output units from getting overloaded with requests, we use multibyte buffers.
--------------
The following are basic interview questions for embedded systems - found http://careerride.com/embedded-systems-interview-for-freshers.aspx
What is lst file?
• This file is also called as list file.• It lists the opcodes ,addresses and errors detected by the assembler.
• List file is produced only when indicated by the user.
• It can be accessed by an editor and displayedon monitor screen or printed.
• Progammer uses this file to find the syntax errors and later fix them.
How is a program executed’ bit by bit’ or’ byte by byte’?
EXAMPLE| ADDRESS | OPCODE | PROGRAM |
| 1 0000 | ORG 0H | |
| 2 0000 | 7D25 | MOV R5,#25H |
| 3 0002 | 7F34 | MOV R7,#34H |
| 4 0004 | 2D | ADD A, R5 |
| 5 0005 | END |
• Firstly,1st opcode 7D is fetched from location 0000 and then the value 25 is fetched from 0001 .
• 25 is then placed in the register R5 and program counter is incremented to point 0002.
• On execution of opcode 7F, value 34 is copied to register R7.
• Then addition of contents of R5 and accumulater takes place.
• Here all the opcodes are 8 bit forming a byte.
Explain DB.
• DB is called as define byte used as a directive in the assembler.• It is used to define the 8 bit data in binary ,hexadecimal or decimal formats.
• It is the only directive that can be used to define ASCII strings larger than two characters.
• DB is also used to allocate memory in byte sized chunks.
• The assembler always converts the numbers lnto hexadecimal.
What is EQU?
• EQU is the equate assmbler directive used to define a constant without occupying a memory location.• It associates a constant value with data label .
• Whenever the label appears in the program ,constant value is subsituted for label.
• Advantage: The constant value occuring at various positions in a program can be changed at once using this directive.
• Syntax: label EQU constant value
How are labels named in assembly language?
• Label name should be unique and must contain alphabetic letters in both uppercase and lowercase.• 1st letter should always be an alphabetic letter.
• It can also use digits and special characters ?,.,@,_,$.
• Label should not be one of the reserved words in assembly language.
• These labels make the progam much easier to read and maintain.
Are all the bits of flag register used in 8051?
• The flag register also called as the program status word uses only 6 bits.• The two unused bits are user defineable flags.
• Carry ,auxillary carry ,parity and overflow flags are the conditional flags used in it.
• PSW.1 is a user definable bit and PSW.5 can be used as general purpose bit.
• Rest all flags indicate some or the other condition of an arithematic operation.
Which bit of the flag register is set when output overflows to the sign bit?
• The 2nd bit of the flag register is set when output flows to the sign bit.• This flag is also called as the overflow flag.
• Here the output of the signed number operation is too large to be accomodated in 7 bits.
• For signed numbers the MSB is used to indicate the whether the number is positive or negative.
• It is only used to detect errors in signed number operations.
Which register bank is used if we use the following instructions
SETB PSW.3 A
SETB PSW.4 B
• Statement A sets 3rd bit of flag register.
• Statement B sets 4th bit of flag register.
• Therefore register bank 3 is initiated .
• It uses memory location 18H to 1FH.
• The register bank is also called as R3.
Issues related to stack and bank 1.
• Bank 1 uses the same RAM space as the stack.• Stack pointer is incremented or decremented according to the push or pop instruction.
• If the stack pointer is decremented it uses locations 7,6,5… which belongs to register bank 0.
• If a given program uses R1 then stack is provided new memory location.
• The push instruction may also take stack to location 0 i.e.it will run out of space.
Explain JNC.
• It is a command used to jump if no carry occurs after an arithematic operation.• It is called as jump if no carry( conditional jump instruction).
• Here the carry flag bit in PSW register is used to make decision.
• The processor looks at the carry flag to see if it is raised or not.
• If carry flag is 0 ,CPU fetches instructions from the address of the label.
Write a program to toggle all bits of P1 every 200ms.
MOV A,#55H
AGAIN: MOV P1,A
ACALL DELAY
CPL A
SJMP AGAIN
DELAY:
MOV R5,#9
HERE1: MOV R4,#242
HERE2: MOV R3,#255
HERE3: DJNZ R3,HERE3
DJNZ R4,HERE2
DJNZ R5,HERE1
RET
• Here the delay produced is 9*255*4MC*90=199,940 micro seconds.
• CPL is used to toggle the bits of P1.
• Short jump is jusd to produce a continuous loop.
Can port 0 be used as input output port?
• Yes, port 0 can be used as input output port.• Port 0 is an open drain unlike ports 2,3,4.
• To use it as input or output the 10k ohm pull-up resisters are connected to it externally.
• To make port 0 as input port it must be programmed by writing 1 to all bits.
• Example:
MOV A,#0FFH
MOV P0,A
Which 2 ports combine to form the 16 bit address for external memory access?
• Port0 and port2 together form the 16 bit address for external memory.• Port0 uses pins 32 to 39 of 8051 to give the lower address bits(AD0-AD7)
• Port2 uses pins 21 to 28 of 8051 to give the higher address bits(A8-A15)
• This 16 bit address is used to access external memory if attached.
• When connected to external memory they cannot be used as input output ports.
Can single bit of a port be accessed in 8051?
• Yes,8051 has the capability of accessing only single bit of a port.• Here only single bit is accessed and rest are unaltered.
• SYNTAX: “SETB X. Y”.
• Here X is the port number and y is the desired bit.
• Example: SETB P1.2
Here the second bit of port 1 is set to 1.
Other than SETB ,CLR are there any single bit instructions ?
• There are total 6 single-bit instructions.• CPL bit : complement the bit (bit= NOT bit).
• JB bit,target: Jump to target if bit equal to 1.
• JNB bit,target: Jump to target if bit is equal to 0.
• JCB bit,target: Jump to target if bit is equal to 1 and then clear bit.
