Program description
What does it do?
The chaser program sweeps a lit LED back and forth using two called subroutines.
New instructions
During this activity, you will learn about these microcontroller instructions:
| call | 'call a subroutine' - save the next program instruction address on the Stack and continue running the program from a label. Like goto, but call is used with return. |
| return | 'return from subroutine' - returns to the address saved on the Stack, continuing the program from the instruction after a call. |
| bcf | 'bit clear file register' - clears a single bit in a file register (RAM). |
| bsf | 'bit set file register' - sets a single bit in a file register. |
| rlf | 'rotate left file register' - shift every bit in a file register (RAM location) one bit to the left. |
| rrf | 'rotate right file register' - shift every bit in a file register one bit to the right. |
Chaser programming activity
Chaser uses decision structures to choose between two subroutines, sweeping an illuminated LED back and forth in the process. It also illustrates subroutine re-use, by using call and return instructions instead of goto.
What you should know before starting
Microcontroller related information
The PIC16F886 microcontroller includes an 8-level hardware Stack which is attached directly to its processing unit (see the simplified PIC16F886 block diagram). This Stack is a LIFO (last-in, first-out) buffer that stores up to eight program addresses.
Certain instructions automatically write or read program addresses to or from the Stack, providing the microcontroller with the ability to store and remember the address of instructions that will be executed following a subroutine or other event (such as an interrupt). Unlike more advanced microprocessors, only the PIC's hardware can control the stack—no program instructions can be used to access the stack.
A call instruction, for example, will store the address of the next physical instruction (ie. the one directly after the call in memory, not the one being called) on to the top of the Stack before jumping to the called subroutine. A return instruction at the end of the called program code will read the top address from the Stack, and reset the program counter with it, effectively allowing the original sequence of instructions to continue from the call.
The advantage of a call instruction over a goto is that call allows a program subroutine to be called from multiple places in the program code, and will always return to the specific subroutine that initiated the call.
Create the program
The entire CHASER.ASM program is shown below. Create a Chaser project in MPLAB, copy this code into it, and build the program.
;CHASER.ASM v2.0 Last modified on August 3, 2010 ;=============================================================================== ;Description: Light chaser sweeps an LED back and forth on the WAND LEDs. ;Start of MPLAB and processor configuration. include "p16f676.inc" ;Include processor definitions __config _CPD_OFF & _CP_OFF & _BODEN_OFF & _MCLRE_ON & _PWRTE_ON & _WDT_OFF & _INTRC_OSC_NOCLKOUT ;End of MPLAB and processor configuration. org 3FFh ;Oscillator calibration location oscillator org 00h ;Start of program memory bsf STATUS,RP0 ;Select memory register page 1 call oscillator ;Store pre-programmed oscillator calibration movwf OSCCAL ;constant in OSCCAL register goto init ;Start program after Interrupt vector org 05h ;Continue after interrupt vector init ;Initialize ports A and C for digital I/O bcf STATUS,RP0 movlw 7 ;Disable comparator and make movwf CMCON ;RA0-2 digital I/O banksel ANSEL ;Switch to register bank 1 movlw 11010111b ;Disable Port A pull-ups, set TMR0 to movwf OPTION_REG ;internal clock with prescaler 256 clrf ANSEL ;Set all PORTA pins to digital movlw 11011111b ;Set RA5 as output and all movwf TRISA ;other PORTA pins as inputs clrf TRISC ;Set all PORTC pins as outputs banksel PORTC ;Return to PORTC register bank clrf PORTA ;Turn off phototransistor LED clrf PORTC ;Turn all LEDs off main bsf PORTC,0 ;Turn on one LED only chaseLeft call timeDelay ;Delay so humans can see the light bcf STATUS,C ;Clear C before rotating rlf PORTC,F ;Shift PORTB contents to the left btfss PORTC,5 ;Has the 1 moved to the left-most LED? goto chaseLeft ;If not, keep moving left ;Otherwise, switch direction chaseRight call timeDelay ;Delay so humans can see the light bcf STATUS,C ;Clear C before rotating rrf PORTC,F ;Shift PORTB contents to the right btfss PORTC,0 ;Has the 1 moved to the far right? goto chaseRight ;If not, keep moving right goto chaseLeft ;Otherwise, switch direction timeDelay movlw 61 ;Preload TMR0 for 50ms time period movwf TMR0 checkTimer movf TMR0,W ;Check TMR0 value btfss STATUS,Z goto checkTimer ;Repeat until TMR0 = 0 return ;Return when done end
Download the program into the CHRP and verify its operation.
Test your knowledge
- The Stack is attached to, and controlled by, the microcontroller. Can your program perform nested calls—a call to a subroutine calls another subroutine—and how many levels deep?
- What do you think will happen if there are more calls than the stack has places for? (Try this in the MPLAB simulator)
- What do you think will happen if a return instruction is encountered before a call? (Try this in the MPLAB simulator)
Apply your skills
- Rotate instructions perform the software equivalent of shift registers and state machines. Modify the Chaser program to continuously cycle a pattern through PORTB. How many states does this pattern have? Can you think of a simple way to expand the number of states in the pattern?
