Pseudocode for interrupt-drive UART Routine on PIC12F752, serving to receive lift fan 
commands as the DOG TREAT for the ME218C project.

Hardware
 
RA0: output, Signal to Lift Fan
RA1: output, LED, GREEN – Lift Fan Status
RA2: output, LED, RED – Receive Status
RA4: output, Reserved for TX  
RA5: input, RX

TIMER1: Rx LED Timer (26ms max, using 26ms)
TIMER2: Baud rate bit timer (26ms max, using 104us)

Inits
 
List the PIC12F752 model for the assembler
Include p12F752.inc
Choose configuration bits
Code Protection OFF
Brownout Detect Enabled
Power Up Timer Enabled
Select Internal Oscillator
 
Variable definitions
 
File memory locations: starting in share RAM at 0x070
WREG_TEMP, STATUS_TEMP, PCLATH_TEMP,
 
Comm Registers
         Byte: RecvShiftCounter
         Byte: RecvShiftRegister
         Byte: RecvDataRegister
 
         Byte: RecvStatus containing the following:
			Bit: RecvStarting         	(0)
			Bit: RecvDataReady          (1)
			Bit: RecvFramingErr         (2)
			Bit: RecvActive      	    (3)

Variable: FanSpeed
         
Defines
BitZero:BitFive numbers
BitZeroHi:BitFiveHi
ALL_BITS_HI equals 8 bits of ones
OneBitTime, HalfBitTime
ADDRESS_MASK,
FullShiftCount
MSB
 
set location 0 in program memory as instruction to go to Main
set location 4 in program memory as instruction to go to ISR
set location 5 in program memory as start of rest of code
 
Label: Main
Change to OSCCON oscillator controller register
Change oscillator to 8MHz
Call InitGPIO
Call InitRxIOC
Call InitRxLEDTimer
Call InitTimer2 for timing the bit time
Clear RecvStarting
Clear RecvFramingErr
Clear RecvDataReady
Clear RecvShiftCounter
 
enable global interrupts
 
Run Loop
Label: Run
Do two no-ops and then goto run to make a nice loop while waiting for
interrupts
 
Label: HUAD
Go to current PC address, i.e. huddle up and die if everything goes wrong
 
Interrupt Service Routine
Label ISR:
Use standard push sequence to save WREG, STATUS, and PCLATH
If IOCIF set (service interrupts for interrupts on change)
If RX interrupt is active
Program Timer2 to fire interrupt ½ bit time
Enable Timer2
Set RecvStarting bit in RecvStatus
Set RecvActive bit in RecvStatus
Set RecvShiftCounter to 10
Clear IOCAF bit associated with RX
Disable IOC on RX
               Endif IOC on RX
               Process other IOC interrupts as needed
Endif (IOCF set)
 
if TMR2 interrupt is active
	if RecvStarting set (doing start bit)
		clear RecvStarting
		if RX is low (we got good start bit)
			program Timer2 to fire interrupt after 1 bit time
			Decrement RecvShiftCounter
		else ( RX is high, so bad start bit)
			program IOC Negative on RX and enable interrupt
			Clear RecvActive bit in RecvStatus
		endif (RX is low)
	else (Doing data bit)
		decrement RecvShiftCounter
		if RecvShiftCounter not 0
			shift RecvShiftRegister 1 position right
			Copy state of RX to MSB of RecvShiftRegister
		else
			copy RecvShiftRegister to RecvDataRegister
			if RX is low (bad stop bit)
				set RecvFramingErr
			endif (RX is low)
			Clear RecvActive bit in RecvStatus
			set RecvDataReady to 1
			Set LastFanCmd equal to RecvDataRegister
			If framing error not equal to 1
					Turn on Rx LED Pulse by calling TurnOnRxLED subroutine
			if LastFanCmd message equal to 0xFF
					 write RA0 and RA1 high
			else
					write RA0 and RA1 low
			endif
			disable TMR2 interrupt
			clear RecvActive bit on RecvStatus
			lower RA2 transmit active LEDs
			program IOC Negative on RX and enable interrupt
		endif (RecvShiftCounter not 0 )
	endif (RecvStarting set)
	clear TMR2IF
endif (TMR2 interrupt )
 
If Timer1 interrupt is active
         Lower RA4 to turn off RxLED
Disable Timer1 with TMR1ON bits of T1CON
         Clear TMR1IF in PIR1
Endif

Interrupt Subroutines
 
TurnOnRxLED
	change to bank 0 with TMR1L and TMR1H
	clear HI 8 bits in TMR1H
	clear LO 8 bits in TMR1L
	Enable Timer1 with TMR1ON bits of T1CON
	change to bank with LATA
	Raise RA4 to turn on RxLED
	return
 
Initialization Routines
Note – Set up Pins initialized to the following:
RA0: output, Signal to Lift Fan
RA1: output, LED, GREEN – Lift Fan Status
RA2: output, LED, RED – Receive Status
RA4: output, Reserved for TX  
RA5: input, RX
 
Label InitGPIO:
	Go to bank containing PortA
	Enable PORTA
	Go to bank containing PortA Data Latch
	clear latch
	Go to bank containing ANSELA
	clear ANSELA to enable digital I/O
	Go to bank containing TRISA
	Prep WREG to set RA<4,2:0> as outputs and set RA<5> as inputs
	move WREG into TRISA
	switch to bank containing LATA
	Set pins RA4,2:0 to be low to start outputs low
	Write to the latch
	return
 
Label InitRxIOC: (to set up RA5 to receive message)
	change to bank containing INCON
	edit interrupt control register to turn on interrupt on change
	set pin RA5to interrupt on falling edges
	change to bank containing IOCAN
	load up 0x20 for RA5
	Return


Label InitRxLEDTimer
    Set Timer1 prescale to 1:8 in T1CKPS bits of T1CON
    Set TMR1H and TMR1L to zero to reset timer
    Enable peripheral by setting TMR1GIE in PIE1
 
Label InitTimer2
	switch to bank containing PIE1
	in PIE1, timer2 interrupt
	switch to bank with T2CON
	prescale of 1:1 & post-scale of 1:1 to achieve 9600 baud, timer off
	move value from WREG into T2CON
	move to bank containing PR2
	move starting value of PR2 to WREG
	write to PR2 reg value of (250)
	Return
 
end to tell assembler we’re done