Tutorial 1 PIC microcontroller

1. What is the difference between microprocessor and microcontroller?

Microcontroller differs from a microprocessor in many ways. First and the most important is its functionality. In order for a microprocessor to be used, other components such as memory, or components for receiving and sending data must be added to it. In short that means that microprocessor is the very heart of the computer. On the other hand, microcontroller is designed to be all of that in one. No other external components are needed for its application because all necessary peripherals are already built into it. Thus, we save the time and space needed to construct devices

2. Draw a block diagram of microcontroller and briefly explain the function of I/O port, timer and ADC?

3. When the microcontroller is reset, what is the value of the program counter holds?

One of the most important effects of a reset is setting a program counter (PC) to zero (0000h), which enables the program to start executing from the first written instruction. 

4. In a serial communication of microcontroller, it needs a protocol. Explain how the protocol works in microcontroller?

The rules of exchange of data are called protocol. Protocol is therefore defined in advance so there wouldn't be any misunderstanding between the sides that are communicating with each other.

Let's suppose we have the following protocol. The logical unit "1" is set up on the transmitting line until transfer begins. Once the transfer starts, we lower the transmission line to logical "0" for a period of time (which we will designate as T), so the receiving side will know that it is receiving data, and so it will activate its mechanism for reception. Let's go back now to the transmission side and start putting logic zeros and ones onto the transmitter line in the order from a bit of the lowest value to a bit of the highest value. Let each bit stay on line for a time period which is equal to T, and in the end, or after the 8th bit, let us bring the logical unit "1" back on the line which will mark the end of the transmission of one data.

5. What is the difference between address bus and data bus?

That "way" is called "bus". Physically, it represents a group of 8, 16, or more wires
There are two types of buses: address and data bus. The first one consists of as many lines as the amount of memory we wish to address, and the other one is as wide as data, in our case 8 bits or the connection line. First one serves to transmit address from CPU memory, and the second to connect all blocks inside the microcontroller.

6. How the data in the memory unit is read and written by the CPU?

Memory consists of all memory locations, and addressing is nothing but selecting one of them. This means that we need to select the desired memory location on one hand, and on the other hand we need to wait for the contents of that location. Beside reading from a memory location, memory must also provide for writing onto it. This is done by supplying an additional line called control line. We will designate this line as R/W (read/write). Control line is used in the following way: if r/w=1, reading is done, and if opposite is true then writing is done on the memory location.

7. As a result of some interference our microcontroller stops executing the program, or worse, it starts working incorrectly. How does the microcontroller overcome this problem?

To overcome this obstacle, we need to introduce one more block called watchdog. This block is in fact another free-run  counter where our program needs to write a zero in every time it executes correctly. In case that program gets "stuck", zero will not be written in, and counter alone will reset the microcontroller upon achieving its maximum value. This will result in executing the program again, and correctly this time around. That is an important element of every program to be reliable without man's supervision

8. PIC memory unit consist of Flash, EEPROM and RAM, explain the role played by each of it.

Program memory (FLASH)- for storing a written program.
Since memory  made in FLASH technology can be programmed and cleared more than once, it makes this microcontroller suitable for device development.

EEPROM - data memory that needs to be saved when there is no supply.
It is usually used for storing important data that must not be lost if power supply suddenly stops. For instance, one such data is an assigned temperature in temperature regulators. If during a loss of power supply this data was lost, we would have to make the adjustment once again upon return of supply. Thus our device looses on self-reliance.

RAM - data memory used by a program during its execution.
In RAM are stored all inter-results or temporary data during run-time.

9. How many clock cycle of oscillator, the free run timer will increase it value by one? What is the maximum value of 8bits free run timer?

FREE-RUN TIMER is an 8-bit register inside a microcontroller that works independently of the program. On every fourth clock of the oscillator it increments its value until it reaches the maximum (255), and then it starts counting over again from zero.

10. “PIC is a RISC microcontroller”, what is the meaning of this statement? How many Instructions available for 16F877?

Microcontrollers with Harvard architecture are also called "RISC microcontrollers". RISC stands for Reduced Instruction Set Computer.

Since PIC16F84 is a RISC microcontroller, that means that it has a reduced set of instructions, more precisely 35 instructions.

11. With the aid of timing diagram, explain what is the meaning of instruction cycle?

Clock from the oscillator enters a microcontroller via OSC1 pin where internal circuit of a microcontroller divides the clock into four even clocks Q1, Q2, Q3, and Q4 which do not overlap. These four clocks make up one instruction cycle (also called machine cycle) during which one instruction is executed.

12. How does the pipelining help to increase the performance of microcontroller?

Instruction cycle consists of cycles Q1, Q2, Q3 and Q4. Cycles of calling and executing instructions are connected in such a way that in order to make a call, one instruction cycle is needed, and one more is needed for decoding and execution. However, due to pipelining, each instruction is effectively executed in one cycle.

13. Why the impulse of reset in PIC needs to last for 72ms?

Following a supply, oscillator starts oscillating. Oscillation at first has an unstable period and amplitude, but after some period of time it becomes stabilized.

That impulse lasts 72ms which is enough time for an oscillator to get stabilized.

14. What are the operations can be performed by ALU?

Arithmetic logic unit is responsible for performing operations of adding, subtracting, moving (left or right within a register) and logic operations.

15. What is status register? Which bits of status register can be affected by ALU instruction execution?

Status Register

As an addition in doing operations in arithmetic and logic, ALU controls status bits (bits found in STATUS register). Execution of some instructions affects status bits, which depends on the result itself. Depending on which instruction is being executed, ALU can affect values of Carry (C), Digit Carry (DC), and Zero (Z) bits in STATUS register.

16. When TRISB register is set, what is the direction of data flow in port B? Write a short program to set four pins as output and four pin as input in port B.

When the TRISB register is set, Port B is set as input, the data will flow into PIC through the pins the Port B.

The following is a short program to set four pins as output and four pin as input in port B.

Bsf     STATUS, RP0  ;Bank1

Movlw  0x0F ;Defining input and output pins

Movwf TRISB ;Writing to TRISB register

17. In PIC, why PORTA pin RA4 is normally used to input signal only?

It is important to note that PORTA pin RA4 can be input only. On that pin is also situated an external input for timer TMR0. Whether RA4 will be a standard input or an input for a counter depends on T0CS bit (TMR0 Clock Source Select bit). This pin enables the timer TMR0 to increment either from internal oscillator or via external impulses on RA4/T0CKI pin.

18. What is the difference between GPR and SFR in term of memory access in PIC?

Locations of RAM memory are also called GPR registers which is an abbreviation for General Purpose Registers. GPR registers can be accessed regardless of which bank is selected at the moment. 

Registers which take up first 12 locations in banks 0 and 1 are registers of specialized function assigned with certain blocks of the microcontroller. These are called Special Function Registers. When different bank is selected, for SPR, we will access to a different memory location even though we use the same register name.

19. How the memory bank can be selected in PIC?

By changing the value of RPO in status register, we can select different memory bank.

Bcf STATUS, RP0   ;Select memory bank 0

Bsf STATUS, RP0   ;Select memory bank 1

20. When microcontroller is interrupted, how the STACK is applied?

PIC16F84 has a 13-bit stack with 8 levels, or in other words, a group of 8 memory locations, 13 bits wide, with special purpose. Its basic role is to keep the value of program counter after a jump from the main program to an address of a subprogram . In order for a program to know how to go back to the point where it started from, it has to return the value of a program counter from a stack. When moving from a program to a subprogram, program counter is being pushed onto a stack (example of this is CALL instruction). When executing instructions such as RETURN, RETLW or RETFIE which were executed at the end of a subprogram, program counter was taken from a stack so that program could continue where was stopped before it was interrupted.

21. Explain the difference between direct addressing and indirect addressing,.

Direct Addressing is done through a 9-bit address. This address is obtained by connecting 7th bit of direct address of an instruction with two bits (RP1, RP0) from STATUS register as is shown on the following picture. Any access to SFR registers is an example of direct addressing.

Indirect unlike direct addressing does not take an address from an instruction but derives it from IRP bit of STATUS and FSR registers. Addressed location is accessed via INDF register which in fact holds the address indicated by a FSR. In other words, any instruction which uses INDF as its register in reality accesses data indicated by a FSR register

22. What is the application of interrupt? How it affect the program flow?

Interrupts are a mechanism of a microcontroller which enables it to respond to some events at the moment they occur, regardless of what microcontroller is doing at the time. This is a very important part, because it provides connection between a microcontroller and environment which surrounds it. Generally, each interrupt changes the program flow, interrupts it and after executing an interrupt subprogram (interrupt routine) it continues from that same point on.

23. Why INTF bit must cleared in interrupt routine?

After interrupt is processed, bit whose setting caused an interrupt must be cleared, or interrupt routine would automatically be processed over again during a return to the main program.

24. What is the function of pre-scalar in PIC? What is the oscillator clock highest divisor can be set by prescaler?

Prescaler is a name for the part of a microcontroller which divides oscillator clock before it will reach logic that increases timer status. Number which divides a clock is defined through first three bits in OPTION register. The highest divisor is 256. This actually means that only at every 256th clock, timer value would increase by one. This provides us with the ability to measure longer timer periods.

25. If 4MHz oscillator is applied to a PIC, what is period per increment in Timer0 status and Watchdog timer status when prescaler rate select bit is set to 111?

For Timer0 :

Period per increment  =  256x(1/4MHz)x4 = 256us

For Watchdog timer:

Period per increment = 128x(1/4MHz)x4 = 128us

State five considerations when selecting microcontroller in embedded system design.

The following items must be taken into consideration

1. Number of I/O pins required
2. Interfaces required
3. Memory requirements
4. Number of interrupts required
5. Real-time considerations
6. Development environment
7. Processing speed required
8. Memory architecture

 What are the consequences of wrong estimation of memory requirement in embedded system design?

Overestimate the memory required may cause unnecessarily expensive solution.

Underestimate the memory required may cause project delay.

 List out five items of hardware design checklist in embedded system design.

•         Verify EPROM, RAM, and peripheral access times. Add wait states if necessary.

•         Verify that setup and hold times are met, both to the peripheral device and to the microprocessor.

•         Make sure crystal type match the system specifications. If  using and external oscillator; be sure it matches duty cycle and drive-level requirements of the microprocessor.

•         Verify that bus loading is not exceeded.

•         Verify that loading of I/O port pins is not exceeded – either sinking or sourcing current.

•         Verify that inputs to timers meet frequency and duty cycle limitations of the part. This usually depends on the microprocessor or timer IC clock speed.

•         Avoids bus contention between peripherals.

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