pic18f assembly example 6 – discrete filter implementation

;-----------------------------------------------------------------------------------------------------------------------------
; This assembly code sets up a timer interrupt to be triggered every millisecond on the pic18f4620.
; When the interrupt service routine is called an ADC on channel 0 is performed and this value  is used as an
;    input to the discrete system given by the difference equation y[n] = x[n]-0.3x[n-1]+0.3y[n-1]-0.75y[n-1]
;    The output of the discrete system is then sent to a  DAC (MCP4921) via the SPI protocol.
;
; Test cicruit used is same as at http://eleceng.dit.ie/daq_lite/build.html
;
; This code is just an educational example and no attempt has been made to optimise it in any way. Use it whatever way you want but I don't accept any responsibility for any problems this code might cause you.
;
; NOTE 1: Only the most significant 8 bits of the 10-bit ADC are used. Therefore the
;    the ADC values used are in the range of 0-255.
; NOTE 2: When implementing the discrete system the ADC values are interpreted as a signed signal which varies between 0.9922 (=127/128) and -1.
;    An ADC value of 255 is interpreted as a signal value of 0.9922; an ADC value of 0 is interpreted as a signal value of -1 ;
;    an ADC value of 128 is interpreted as a signal value of 0; An ADC value of 127 is interpreted as a signal value of -0.0078 (1/128)
; NOTE 3: The pic18f does not have a floating point unit and signed decimal values are represented in and 8-bit, fixed point, 2's complement format.
;    A signal value of 0.9922 is stored as a binary value of '01111111' in memory 
;    A signal value of -1 is stored as a binary value of '10000000'  in memory
;    A signal value of 0.5 is stored as binary value of '01000000' in memory 
;    A signal value of -0.5 is stored as binary value of '11000000' in memory 
;    A signal value of 0.75 is stored as binary value of '01100000' in memory 
;    A signal value of -0.75 is stored as binary value of '10100000' in memory 
;    A signal value of 0.0078 is stored as binary value of '00000001' in memory
;    A signal value of -0.0078 is stored as binary value of '11111111' in memory 
;
; by David Dorran (https://dadorran.wordpress.com) May 2014
;-----------------------------------------------------------------------------------------------------------------------------
 

;configure the  assembler directive 'list' so as to set processor to 18f4620 and set the radix used for data expressions to decimal (can be HEX|DEC|OCT)
    list p=18f4620, r=DEC
    #include <p18f4620.inc>
; configure the micro so that the watchdog timer is off, low-voltage programming is off, master clear is off and the clock works off the internal oscillator
    config WDT=OFF, LVP=OFF, MCLRE=OFF, OSC=INTIO67
;The org directive tells the compiler where to position the code in memory
    org 0x0000 ;The following code will be programmed in reset address location i.e. This is where the micro jumps to on reset
    goto Main ;Jump to Main immediately after a reset

 ;NOTE: When an interrupt is triggered the micro jumps to 0x0008 for high priority and 0x0018 for low priority.
    org 0x0008
    goto hi_isr ;once a high priority interrupt is triggered jump to hi_isr
    org 0x0018
    goto low_isr ;once a low priority interrupt is triggered jump to low_isr

;--------------------------------------------------------------------------
; Main Program -------------------------------------------------------------
;--------------------------------------------------------------------------
    org 0x0100 ; the following code is placed at address 0x0100 in program memory (Flash memory)
Main
    ; store the filter variables in a contiguous block in RAM starting at address 0x000
    cblock 0x000
    ; y_1 = y[n-1]; y_2 = y[n-2]; x_1 = x[n-1]; x_2 = x[n-2]
    x, y,a1,a2,b0,b1,b2, var1,var2, x_1, y_1, x_2, y_2
    endc

    ; set up b and a coefficients of the discrete system (y[n] = x[n]-0.3x[n-1]+0.3y[n-1]-0.75y[n-1])
    movlw .127
 	movwf b0 ; approx = 1 (exact value is 0.9922)
    movlw B'11011010'
 	movwf b1 ; approx = -0.3 (exact value is -0.3047) 
    movlw B'00000000'
 	movwf b2 ; = 0
    movlw B'00100110'
 	movwf a1 ; = approx 0.3 (exact value is 0.3047)
    movlw B'10100000'
 	movwf a2 ; = -0.75

    ; intialise the 'past value' variables to zero
    clrf y_1
    clrf y_2
    clrf x_1
    clrf x_2

    call micro_config ; configure pins,timers, ADC etc.

main_loop
    nop ; Just wait for a timer interrupt to be triggered
    goto main_loop

    
    ;----------------------------------------------------------------------------------------------------------
    ;-------multiply accumulate - multiply var1 by var2 and accumulate result in y   --------------------------
    ;------   this routine is used a lot to filter the signal -------------------------------------------------
    ;----------------------------------------------------------------------------------------------------------
mac
    ; pic18f only has 8X8 unsigned multiply - the following code analyses the result of an 8X8 unsigned multiply to produce a signed result
    movf var1, W
    mulwf var2 ; var1 * var2 -> PRODH:PRODL (var1 and var interpreted as     btfsc var2, 7 ; Test Sign Bit (NOTE: if var2 is negative (2's complement representation) then its unsigned interpretation is 256-var2 and then the result stored in PROD is var1*(256-var2) = var1*256-var1*var2
    btfsc var2, 7 ; Test Sign Bit
    subwf PRODH, F ; PRODH = PRODH - var1 (equivalent to PROD = PROD - var1*256)
    movf var2, W
    btfsc var1, 7 ;Test Sign Bit
    subwf PRODH, F ; PRODH = PRODH - var2

    ; The two most significant bits of PROD are sign bits so shift PRODH to the left and grab the most significant bit of PRODL to get the 8 most significant useful bits.
    rlncf PRODH,0 ; W contains PRODH shifted to the left by one
    movwf var2 ; using var2 as it is available (not for any other reason)
    bsf var2, 0
    BTFSS PRODL,7 ; set the most significant bit of PRODL as the least signifcant in W
    bcf var2, 0
    movf var2,W

    addwf y,f ; accumulate the result of multiplication in y. Note not handling overflow. Would also be better to accumulate those 7 bits discarded after the multiplication and round afterwards for a more accurate result

    return

    
    ;----------------------------------------------------------------------------------------------------------
    ;-----High Priority Interrupt Service Routine -------------------------------------------------------------
    ;----------------------------------------------------------------------------------------------------------
    org 0x0200
hi_isr
    btfss INTCON, TMR0IF
    goto end_int
    bcf INTCON, TMR0IF; reset interrupt
     ;An interrupt has been set up to be generated whenever TMR0 goes from FFFFh to 0000h i.e. on overflow.
    ;Would like the timer overflow flag triggered every 1ms so will set timer to 0xffff - 1000 = 0xFC17
    movlw 0x17
    movwf TMR0L
    movlw 0xFC
    movwf TMR0H

	bsf ADCON0,GO; start ADC conversion

    ; wait for hardware to reset GO bit - indicating ADRES SFR is ready to read
adc_not_ready
	btfsc ADCON0, GO
    goto adc_not_ready

    clrf y

    movf y,W

    movf ADRESH,W
    addlw -128 ; account for dc offset. The input values from ADRESH will be in range 0-255. These values represent a signal that varies betwen 1 and -1: the ADC value of 255 is mapped to 1 and ADC value of 0 is mapped to -1; ADC value of 128 is mapped to 0.

    movwf x
    movwf var1
    movff b0,var2
    call mac ; multiply var1 by var2 and accumulate the result in y (mac - multiply accumulate)

    movff x_1, var1
    movff b1, var2
    call mac;

    movff x_2, var1
    movff b2, var2
    call mac;

    movff y_1, var1
    movff a1, var2
    call mac;

    movff y_2, var1
    movff a2, var2
    call mac

    ; update past values of the inputs and outputs to use on next iteration of the discrete system
    movff y_1, y_2
    movff y, y_1
    movff x_1, x_2
    movff x, x_1

    movf y,W
    addlw .128 ; account for dc offset
    movwf y
    goto spi_write ;write contents of y to SPI device
end_int
    retfie ;return and reset interrupts

    
    
    ;----------------------------------------------------------------------------------------------------------
    ;-------configure pins, ADC,timers and interrupts on the pic18f4620    -----------------------------------
    ;----------------------------------------------------------------------------------------------------------
micro_config
     ; configure LATD0-LATD3 as outputs and RD4-RD7 as inputs
    movlw 0xf0
    movwf TRISD
    clrf LATD ; turn off all LATD output pins

    ; SPI configuration ------------------------------------------------------------
    clrf TRISC
    ;bcf TRISC,3 ; pin 3 on PORTC used as SPI clock and should be configured as an output to operate in master mode
    ;bcf TRISC,5 ; pin 5 on PORTC used for SPI serial data out (SDO) and should be configured as an output
    movlw B'10110000' ; set up SPI control register see pg 163 for details
    movwf SSPCON1;
    bsf SSPSTAT,CKE; data transmitted on rising edge
    ; END SPI config ----------------------------------------------------------


    ; Set clock frequency (section 2 of the PIC18F4620 Data Sheet)
    ; Set Fosc = 8MHz, which gives Tcy = 0.5us
    movlw B'01110000'
    movwf OSCCON

    ; all channels set up for ADC - no particular reason why
    movlw B'00000001'
 	movwf ADCON1

    ; set tad so that capacitor on ADC can fully charge - see pg129
    movlw B'00100010'
 	movwf ADCON2

    clrf ADCON0; select analog channel 0;
    bsf ADCON0,ADON; //enable ADC

    bcf INTCON, GIE ;Disable global interrupts
    bcf T0CON, TMR0ON; turn off timer 0
    bsf INTCON, TMR0IE; Enable TIMER0 interupt

    ;set up 1:2 prescaler so that TMR0 SFR is incremented every 2 Tcy  i.e. 1us
    bcf T0CON,0
    bcf T0CON,1
    bcf T0CON,2

    bcf T0CON, T0CS; use internal instruction cycle clock
    bcf T0CON, T08BIT; use 16 bit mode
    bcf T0CON, PSA; turn on prescaler

    ; setup so that timer0 overflow is triggered quickly initially
    ; after the first trigger then set it up so that it is triggered at the rate you want in the high_isr routine
    movlw 0xFF
    movwf TMR0L
    movlw 0xFF
    movwf TMR0H

    bsf INTCON2, TMR0IP; ; Set the timer0 interrupt up as high priority
    bsf INTCON, GIE ;Enable global interrupts
    bsf T0CON, TMR0ON; turn on timer 0
    return

    ;----------------------------------------------------------------------------------------------------------
    ;-----WRITE contents in y to DAC (MCP4921) using SPI ------------------------------------------------------
    ;----------------------------------------------------------------------------------------------------------
spi_write
    ;two bytes of data sent to spi daq (a 12-bit dac). First nibble of first byte is dac config settings; second nibble is
    ; the most significant 4 bits of 12bit numerical value. The second byte contains the 8 least signifcant bits of the 12-bit value

	bcf LATD, LATD1; // Select SPI DAC chip

    movf SSPBUF, W ;WREG reg = contents of SSPBUF (this clears BF) which will be set again after the data is transmitted
    ; load SSPBUFF with spi dac config data to be sent to SPI device
    ; First nibble of first byte sent to dac is dac config settings; second nibble is
    ; the most significant 4 bits of 12bit numerical value
    swapf y, f
    movlw 0x0f
    andwf y, W ; W no holds 4 most signicant bits in lower nibble

    iorlw 0x70 ; these are configuration bits for spi dac
    movwf SSPBUF

spi_wait1
    btfss SSPSTAT, BF ;Has data been received (transmit complete)?
    goto spi_wait1 ;No

    movf SSPBUF, W ;WREG reg = contents of SSPBUF (this clears BF) which will be set again after the data is transmitted
    ; load SSPBUFF with y data to be sent to SPI device

    movlw 0xf0
    andwf y, W ;
    movwf SSPBUF

spi_wait2
    btfss SSPSTAT, BF ;Has data been received (transmit complete)?
    goto spi_wait2 ;No

	bsf LATD, LATD1; // Select SPI DAC chip
    goto end_int


    ;----------------------------------------------------------------------------------------------------------
    ;-----Low Priority Interrupt Service Routine -------------------------------------------------------------
    ;----------------------------------------------------------------------------------------------------------
    org 0x0300
low_isr
    nop ; put whatever you would like to do on low priority interrupt here
    retfie ;return and reset interrupts

    end ; End of ASM code