Generating a Sawtooth Signal with Programmable Ramp Generator

[William Roth]

This example code demonstrates how a PIC with a Programmable Ramp Generator (PRG) peripheral can produce a sawtooth signal and route the output to an I/O pin. A PIC 16F1769 was used, but any PIC with a PRG peripheral should work in a similar manner.

PIC chips with Core Independent Peripherals (CIPS) allow tasks to be done with out taking up valuble core resources. In this example the following CIPS are used. Timer2, DAC1, OPA0, OPA1, PWM3 & PRG0. No external components are required.

The code below will generate a 38KHz sawtooth signal on Pin 7. The signal is referenced to 0V via the DAC a DAC level of 0. The rate of rise is .2V/us . The peak amplitude is 4.2V

By adjusting PWM, DAC Level, PRG Mode & rise/fall rates, a sawtooth, falling ramp, or triangle signal can be generated at various frequencies and rising/falling rates.

Here is the code to get started. I used a 4x 20 LCD display for code development/debugging. The Printbits sub displays a register in binary format. Example: Printbits PRG1CON0,0. This will print the register bits on line 0 of the LCD Display.

'''A demonstration program for GCB.
'''---------------------------------------------------
'''Program to Demo Programable Ramp Generater
'''William Roth
'''12.01.2016
'''****************************************************

  #chip 16F1769, 32
  #config OSC = INTOSC, MCLRE = ON, WDTE = OFF, PPS1WAY = OFF
  #OPTION Explicit

''   'Using SWI2C for Display
  #define I2C_MODE Master
  #define I2C_DATA PORTB.4
  #define I2C_CLOCK PORTB.6
  #define I2C_DISABLE_INTERRUPTS ON
  #define LCD_IO 10
  #define LCD_I2C_Address_1 0x4E

  #define FOSC 1
  #define FOSC4 2

  CLS
  PRINT "Great Cow Basic"
  Locate 1,0
  Print "PRG Demo"
  Locate 2,0
  Print ChipNameStr
  wait 1 S

INIT:
  INIT_PWM3    ;Setup PWM3
  START_PWM3   ;Start the PWM
  INIT_DAC1    ;Setup DAC1
  INIT_OPA1    ;Setup OPAMP1
  INIT_OPA2    ;Setup OPAMP2
  INIT_PRG1    ;Setup PRG1

MAIN:

  START_PRG1      ;  GO!

'---- Main loop ----
Do

    'Main program goes here

LOOP

'------- Sub Routines ----------
Sub INIT_DAC1
  DAC1CON0 = b'00000000'    ; Defaults
  DAC1REFH = b'00000000'    ;
  DAC1REFL = b'00000000'    ;
  SET DAC1CON0.5 OFF        ; DAC1 is on NO Output on RA0
  SET DAC1CON0.7 ON         ; ENABLE DAC1
  SET DAC2LD ON             ; Write DAC Val to Buffers
End Sub

Sub INIT_OPA1
  OPA1PCHS = b'00000010'    ; POS INput is DAC1 OUT
  OPA1CON  = b'10010000'    ;Enable with Unity Gain (Buffer)
End Sub

Sub INIT_OPA2
  OPA2PCHS = b'00001000'    ;POSIN = PRG1 OUT
  OPA2CON  = b'10010000'    ; Enable with Unity Gain
  ;PRG Output will be on RC3 Pin7
End Sub

Sub INIT_PRG1
    ;Setting up Ramp Generator as Rising Ramp
    ;DAC1 / OPA1 as Reference Supply source
    ;PWM3 as timing source
    SET PRG1CON0.7 ON   'Enable Module

    Do while PRG1CON1.2 = 0 'wait for PRG Ready
    Loop

    SET PRG1CON0.3 ON   ;Mode = Rising
    SET PRG2CON0.2 OFF

    SET PRG1CON0.5 ON   ;Edge Triggered
    SET PRG1CON0.4 ON

    SET PRG1CON0.1 ON   ;One shot enabled (See Datasheet)

    SET PRG1CON1.1 ON   ;  Set Polarity
    SET PRG1CON1.0 OFF  ;  Set Polarity

    PRG1CON2 = b'00000000'   ; Slope rate default .2 us/v
    PRG1FTSS = b'00000101'   ; Falling Timing Source PWM3
    PRG1RTSS = b'00000101'   ; Rising  Timing source PWM3
End Sub

Sub START_PRG1
    Set PRG1CON0.0 ON    ; GO!  Start the PRG
End Sub

Sub START_PWM3
     Set TMR2IF OFF          ; Clear the bit
     Set T2CON.7 ON          ;Start the timer
     PWM3CON = b'10000000'   ; Start the PWM
End Sub

Sub Stop_PWM3
     PWM3CON = b'00100000'    ; Stop the PWM
     Set TMR2IF OFF          ; Clear the bit
     Set T2CON.7 OFF         ;Start the timer
     TMR2 = 0
End Sub

Sub INIT_PWM3
     T2PR = 200            ;Default is 255 at POR
     T2CLKCON = FOSC4
     T2CON = b'00100000'   ;Set TMR2 Prescale
     PWM3CON = 0           ; Clear
     PWM3DCH = 175         ; High bits
     PWM3DCL = 0           ; Low Bits
 End Sub

Sub printbits(databyte,LCD_line)
  Locate LCD_line,0
  Print "                    " 'clear the line
  locate LCD_line,0
  Print databyte.7 : Print " "
  Print databyte.6 : Print " "
  Print databyte.5 : Print " "
  Print databyte.4 : Print " "
  Print databyte.3 : Print " "
  Print databyte.2 : Print " "
  Print databyte.1 : Print " "
  Print databyte.0 : Print " "
end sub
end

[stan cartwright]

Could be useful to control x/y scope and bresenham's line algo to do vector graphics like asteroids was , 2 ramp generators clocked at different frequency.