Operational Advanced Embedded Systems Interview Preparation Guide

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Microprocessor is managers of the resources (I/O, memory) which lie outside of its architecture

Microcontroller have I/O, memory, etc. built into it and specifically designed for control

Interrupt latency is a time taken to return from the interrupt service routine post handling a specific interrupt. By writing minor ISR routines, interrupt latency can be reduced.

☛ 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.

☛ 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.

☛ 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 arithmetic operation.

☛ Atmel Corporation is manufacturing flash ROM
☛ At 89C51 chip of atmel is using flash ROM.
☛ Here no Rom eraser is needed.
☛ Erasing process is done by the PROM burner itself.
☛ To avoid use of PROM burner the 8052 version support in-system programming.

Model transformations involve multiple models that are used to define different views of a system. It provides different level of granularity that it doesn’t use either the top-down approach or the bottom-up approach to implement the basic functionality of the system. It is used to integrate the library components used that involves the iteration of the model that needs to be constructed. It also involves the analysis of the model so that the process can be made automated by using the construction tools. The compilation made the progress by improving the code that is written in high level language and the code generator produce the code that is required for the machine language.

For embedded system, the buses used for communication includes

I2C: It is used for communication between multiple ICs
CAN: It is used in automobiles with centrally controlled network
USB: It is used for communication between CPU and devices like mouse, etc.
While ISA, EISA, PCI are standard buses for parallel communication used in PCs, computer network devices, etc.

Real-time embedded systems are computer systems that monitor, respond or control an external environment. This environment is connected to the computer system through actuators, sensors, and other input-output interfaces.

☛ 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.

☛ 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

☛ 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(Boolean True) 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 pre-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.

☛ 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.

A semaphore is an abstract datatype or variable that is used for controlling access, by multiple processes to a common resource in a concurrent system such as multiprogramming operating system. Semaphores are commonly used for two purposes

☛ To share a common memory space
☛ To share access to files

☛ 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.

☛ 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.

☛ Ultraviolet EPROM is used in the version 8751.
☛ Here it becomes easier to erase the data using uv rays.
☛ It takes around 20 minutes to erase the data.
☛ To overcome this short coming flash ROM versions of 8751 available.

Input and output devices or functions allow the user to interact with the external files. Input and output functions are used to transfer the load on the bus. It uses the multiplexes having the input and output signals that remain same. Input and output buses move at the slower rate or speed than the processor speed. This increases the problem of bottleneck or the deadlock due to poor performance. There is a possibility to send more transistors for a layout to be given. These different devices may have very different speeds of communication. When programming IO bus control, make sure to take this into account.
In some systems, memory mapped IO is used. In this scheme, the hardware reads its IO from predefined memory addresses instead of over a special bus. This means you'll have simpler software, but it also means main memory will get more access requests.

Timers in embedded system are used in multiple ways

☛ Real Time Clock (RTC) for the system
☛ Initiating an event after a preset time delay
☛ Initiating an even after a comparison of preset times
☛ Capturing the count value in timer on an event
☛ Between two events finding the time interval
☛ Time slicing for various tasks
☛ Time division multiplexing
☛ Scheduling of various tasks in RTOS

Essential components of embedded system includes

☛ Hardware
☛ Processor
☛ Memory
☛ Timers
☛ I/O circuits
☛ System application specific circuits
☛ Software
☛ It ensures the availability of System Memory
☛ It checks the Processor Speed availability
☛ The need to limit power lost when running the system continuously
☛ Real Time Operating System
☛ It runs a process as per scheduling and do the switching from one process to another

☛ 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 program much easier to read and maintain.

1. 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.
2. External signals, some condition in the program or by the occurrence of some event, these could be the reasons for generation of an interrupt.
3. Interrupts can also be masked so as to ignore them even if an event occurs for which a routine has to be executed.
4. 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.

☛ The port 3 can be used as simple input /output port or provides signals like interrupts.
☛ P3.0 and p3.1 are used for serial communication.
☛ P3.2 and P3.3 are used as external interrupts.
☛ P3.4 and P3.5 are used for timers.
☛ P3.6 and P3.7 are used as read write signals of external memory.

☛ Hardware design is designed with the collaboration of interconnected parallel components that inherits the properties of each other. Whereas, Software design is designed with sequential components, that are based on objects and threads.

☛ Hardware design structure doesn’t change dynamically and it can’t be created, modified or removed easily. Whereas, Software design structure can be changed dynamically and re-usability features, used to define the components. It also includes easy creation, modification and removal of the components from the software.

☛ Hardware design focuses on individual components that are represented using analytical model that uses the transfer functions. Whereas, Software design represent the components using computation model that can have abstract execution engine or it can use the virtual machine that are non-deterministic.

The interrupts that comes in between the input/output operations gets detected when the input/output devices are ready. The interrupt never gets handled directly rather, it sends the interrupt signal to the thread to the input/output device that is ready to allow the thread to take necessary actions. The thread uses the signaling concept that allows the initialization to be done using the semaphore that keeps the states updated and handle the interrupt in an easy way. The input/output device getting the request and it also passes the semaphore to handle. The input/output device takes the semaphore that is ready. The thread is having the minimum latency that uses the first level interrupt handler to handle the interrupts completely. It allows the priority of the thread to be set and it doesn’t allow the context to be change as well.