PIC Discussion By Eng. Tamar Jomaa

Transcription

1 PIC Discussion By Eng. Tamar Jomaa 1

2 Write assembly language instructions to clear the general purpose registers of PIC16F84A microcontroller (don t write the whole program) 2

3 Islamic university Electrical engineering PIC course Chapter#2 Programming Microcontroller Using Assembly Language 3

4 Outlines for part#2: 2.4 Delay 2.5 Interrupts 2.6 TMR 4

5 2.4 Delay In this section we will learn how building delays. A delay is a mechanism that keeps the processor from executing a part of its program for a specific amount of time. Methods for creating these delays are: "NOP" Delay : A simple delay can be achieved by throwing in a few instructions that don't do anything useful except waste machine cycles. Basic Loop Delay: The next example show how to make delay for 41 cycles. 5

6 Example#1: 2.4 Delay 1 1 1x1 1x9+2 Will be evaluated one cycle Will be evaluated one cycle Take one cycle but it repeat 1 times Decrease from 9 to 1 take one cycle repeat 9 times. When decrease from 1 to a skip will be happen, so take 2 cycles. Take 2 cycles repeat 9 times Total delay = (1+1+1x1+1x9+2+2x9)cycle= 41 cycle Note: 1cycle=4Tosc=4x(1/Fosc)=4/(4MHz)=1µsec. so total delay=41µsec 6

7 2.4 Delay Example#2 :is a simple example consist of connecting a led to RB, then turning on/off this led in a flasher way: 7 Outer loop Inner loop 1 W PORTB 1 XORWF PORTB, F Delay 1 1 W PORTB XORWF PORTB, F

8 2.4 Delay 1 1 1x255 1x255 1x255 1x255x255 (1x254+2)x x255x2 1x x254 2 Note: ODH register will decrease from 255 to then it goes out from LOOP. Then the OCH register will decrease by 1 then the ODH register will full again in then begin decrease and so on The inner loop will repeat 255x255, but the outer loop will repeat only

9 2.4 Delay Total delay =[1+1+1x255+1x255+1x255+1x255x255+(1 x254+2)x x255x2+1x254x2+2x254 +2]x(4/(4MHz))=.26sec Important note: This method of delay is not sufficient especially at long delay so we learn another method by using TMR. 9

10 Example#3: 2.4 Delay Write a PIC subroutine in assembly language to give a fixed delay of 2us. Assume that a 4MHz oscillator is being used X(K-1) 2 2X(K-1) 2 Total delay= 1+1+(K-1)+2+2X(K- 1)+2=3K+3 Cycles (3K+3)µsec=2µsec K=65 1

11 2.5 Interrupts 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. 11

12 2.5 Interrupts Sources of interrupt: In the PIC16F84A, there are four sources of interrupt: Internal interrupt Termination of writing data to EEPROM. TMR interrupt caused by timer overflow. External interrupt Interrupt during alteration on RB4, RB5, RB6 and RB7 pins of port B. External interrupt from RB/INT pin of microcontroller. 12

13 2.5 Interrupts Registers used with interrupt: 1. Control interrupt register (INTCON) at Bh address. 2. Option register at 81h address. 3. Control EEPROM register (EECON 1) at 88h address. The most important register is (INTCON), so you can see the datasheet to know more about this register. 13

14 2.5 Interrupts flags 14

15 2.5 Interrupts 15

16 2.5 Interrupts 16

17 2.5 Interrupts The bellow Figure shows the most important interrupt register INTCON, which controls the usage and selection of interrupt sources in the PIC16F84A. 17

18 2.5 Interrupts We can use the bits of INTCON register to make interrupt as follow: 1) First we set GIE bit which must be enabled for any interrupt. 2) Second we enable one of this bits( TIE,EEIE, RBIE,INTE) as we like to use the interrupt: If we use external interrupt, we set INTE to 1. If we use TMR interrupt, we set TIE to 1. If we use RB interrupt, we set RBIE to 1. If we use EEPROM interrupt, we set EEIE 1. 3) When interrupt occurred, the one of flag bits (INTF, RBIF, TOIF) is set physically so it must be cleared in software. 18

19 2.5 Interrupts Keeping the contents of important registers: An important part of the interrupt process is the stack. 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. 19

20 2.5 Interrupts 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 from where it stopped before the interrupt. These operations of placing on and taking off from a program counter stack are called PUSH and POP. The step of push and pop is performed because the contents of the most important registers may change during the interrupt. The most important registers we must keep is the work register and status register and this is done by storing the work register in any bank and storing the status register in bank. 2

21 2.5 Interrupts After writing the main body of the interrupt and we can restore the status register then restore the W register. 21

22 2.5 Interrupts The following example shows how to use the external interrupt to turn on a led connected to RB2 when a pushbutton connected RB (External interrupt source) is pressed 22

23 2.5 Interrupts 23

24 2.5 Interrupts 24

25 problem#1: using external interrupt INT to implement a counter to counts from to 1111 at falling edge input. 25

26 problem#2: using external interrupt INT to implement a counter to counts from 1111 to at RISING edge input 26

27 2.6 TMR Timer is an internal 8bit register that increments automatically with every PIC instruction cycle until the count over flows timer capacity. This takes place when the timer count goes from xff to x. At that time,the timer restarts the count. The timer has the following characteristics: A timer register that is readable and writeable by software. Can be powered by an external or internal clock. Timing edge for external clock can be selected. 8-bit software programmable prescaler. Interrupt capability. Can be used as a timer or as a counter. 27

28 2.6 TMR Two register are used for control timer: 1) INTCON register: is used in timer mode to enable timer interrupt as showed in pervious section. 2) OPTION register: is used for control timer operation as select mode, postescaler for timer or WDT timer,select the value of prescaler and counter work in fall edge or high edge. 28

29 2.6 TMR Select mode:to choose timer work on timer mode or counter mode, we use bit5(tcs). If it is set,timer take external clock ( work as counter). Else it take internal clock & work as timer. Select edge: If trigger TMR was enabled with impulses from a RA4/TCKI pin, bit 4(TSE) would determine whether it would be on the rising or falling edge of a signal. 1= Falling edge = Rising edge Prescale: To select prescale for a WDT timer or for TMR we use bit3 (PSA): 1=prescaler is assigned to WDT. =prescaler is assigned to free timer TMR The counter prescaler consists of the three low-order bits in the OPTION register. These bits allow selecting eight possible values that serve as advisor for the counter rate.when the prescaler is disabled, the counter rate is one-fourth the processor s clock speed.if the prescaler is set to the maximum value (255) then one of 255 clock. Signals actually reach the timer. 29

30 2.6 TMR Table below shows the prescaler settings and their action on the rate of thetimer module and the Watchdog Timer: 3

31 Delay with TIMER: 2.6 TMR A general formula for calculating the number of timer beats per second is as follows : T =C/(4*P*R) Where T: is the number of clock beats per second C :is the system clock speed in Hz P: is value stored in the prescaler R: is the number of iterations counted in thetmr register. The range of both P and R in this formula is 1 to 256. Example how to make delay with timer: 31

32 Counter mode: 2.6 TMR The PIC16F84A can be programmed so hat portra4/tcki is used to count events or pulses by initializing the Timer module as a counter. Without interrupts, the process requires the following preparatory steps: Port-A, line4, (RA4/TCKI) is defined for input. The Timer register (TMR) is cleared. The OPTION register bits PSA and PS:PS2 are initialized if the prescaler is to be used. The OPTION register bit TSE is set so as to increment the count on the high-to-low transition of the port pin if the port source is active low.otherwise the bit is cleared. The OPTION register bit TCS is set to select action on the RA4/TCKI pin. 32

33 2.6 TMR The following example is using TMR as a counter to count from to 99 as shown in Figure: 33

34 34

35 2.6 TMR problem#1: using interrupt technique with tmr to implement 15 usec delay 35

36 Homework: Using interrupt technique with tmr to implement 15 msec delay Delivery date: next discussion lecture. 36

37 Be free to ask any question 37