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.

Program requirements

To use this program you will need:

An assembled CHRP board, microcontroller, and power supply, a programming cable, and a Windows PC with the MPLAB IDE software and downloader software as described in the Output activity.

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 December 1, 2008
;Description:	Back and forth light chaser using the PORTB LEDs.

;Start of MPLAB and processor configuration.

	include	""		;Include processor definitions 

	__config _CONFIG2, _WRT_OFF & _BOR40V

;End of MPLAB and processor configuration.

	org	00h			;Start of program memory

		clrf	PORTA		;Turn off all port outputs
		clrf	PORTB
		clrf	PORTC
		goto	initPorts	;Jump to initialize subroutine

	org	05h

initPorts	;Set Ports B and C to support CHRP digital circuitry.

		banksel	ANSELH		;Switch register banks
		movlw	01010111b	;Enable Port B pull-ups, TMR0 internal
		movwf	OPTION_REG	;clock, and 256 prescaler
		clrf	ANSELH		;Set all PORTB pins to digital
		clrf	TRISB		;Set all PORTB pins as outputs for LEDs
		banksel	TRISC		;Switch register banks
		movlw	10110000b	;Setup serial input and output pins,
		movwf	TRISC		;and set motor outputs
		banksel	PORTB		;Return to PORTB register bank

main		bsf	PORTB,0		;Turn on one LED only

chaseLeft	call	timeDelay	;Delay so humans can see the light
		bcf	STATUS,C	;Clear C before rotating
		rlf	PORTB,F		;Shift PORTB contents to the left
		btfss	PORTB,7		;Has the 1 moved to the far left?
		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	PORTB,F		;Shift PORTB contents to the right
		btfss	PORTB,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

	org	1F00h			;Start of bootloader code area
	res	256			;Reserve memory for bootloader


Download the program into the CHRP and verify its operation.

How the program works

The start of the program and port initialization follow the same pattern as in the Count program.

Following the port initialization, the bsf PORTB,0 instruction beside the main label lights up one PORTB LED by setting the bit in position 0 of the PORTB register. Whereas the combination of movlw and movwf instructions used in Output and the initPorts subroutine let us change eight bits at a time, bsf changes only one, specified bit.

main		bsf	PORTB,0		;Turn on one LED only

chaseLeft	call	timeDelay	;Delay so humans can see the light
		bcf	STATUS,C	;Clear C before rotating
		rlf	PORTB,F		;Shift PORTB contents to the left
		btfss	PORTB,7		;Has the 1 moved to the far left?
		goto	chaseLeft	;If not, keep moving left
					;Otherwise, switch direction

The first line of chaseLeft calls the timeDelay subroutine. Like goto, a call instruction changes the flow of the program. Unlike goto, call stores its return address on the stack so that a later return instruction (in the called subroutine) can resume program execution from the line directly after the call. This important difference between call and goto allows both the chaseLeft and chaseRight subroutines to call the same timeDelay subroutine and in each case return to the appropriate calling routine.

The timeDelay subroutine is essentially the same as the delay code in the Count program, with the addition of a return instruction at its end. After TMR0 reaches zero, return causes the microcontroller to return to the instruction directly below call timeDelay.

After returning from the time delay, the bcf STATUS,C instruction clears the C (Carry) bit in the Status register. Carry needs to be cleared before the upcoming rotate instruction that will be used to move the LED pattern. During a rotate operation, the contents of a file register are rotated (shifted by one bit) through C and back into the opposite side of the register. Clearing C before the rotate ensures that it won't accidentally introduce a second lit LED into our display pattern.

Next, rlf PORTB,F shifts the contents of Port B (the light pattern in this program) one position to the left. The left-most bit (bit 7) in the register rotates into Carry, and Carry rotates back into bit 0 of the register—which, again, is the reason we had to clear Carry before rotating.

Then the decision-making btfss PORTB,7 instruction checks to see if the one we loaded into the right side of the PORTB register has arrived in the left-most bit position (the bit 7 position) by checking if bit 7 is set. If bit 7 is clear, the program executes the goto chaseLeft instruction and repeats the time delay before rotating Port B again. After seven rotates, when bit 7 is finally set, program execution will skip over the goto chaseLeft instruction and continue into the chaseRight subroutine.

chaseRight performs the exact same function as chaseLeft, albeit in the opposite direction, and both subroutines are able to use the same timeDelay code through their respective call instructions.

Test your knowledge

  1. 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?
  2. What do you think will happen if there are more calls than the stack has places for? (Try this in the MPLAB simulator)
  3. What do you think will happen if a return instruction is encountered before a call? (Try this in the MPLAB simulator)

Apply your skills

  1. 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?