ADC and PWM on PIC16F88

Demo project to read potentiometer value using ADC and output PWM to LED. I use it to learn how to use ADC and PWM on PIC16F88.

What it does (in a loop, around every 10ms):

1. Read potentiometer value using ADC, on RA0/AN0/PIN17, ADC produces 10-bit value from 0 to 1023 (0x3ff)
2. set LED brightness on RB0/INT/CCP1/PIN6 using PWM - duty based on ADC value, PWM frequency is fixed at 1 kHz
3. Blinks LED on RA1/AN1/PIN18, period based on ADC value, the shortest period is 50ms, longest is 1s

Used HW:

• PICDEM Lab Development Kit
• PIC16F88, note¹ - included with latest series of DM163045

Schematic is below:

Important! These pins should be left not-connected for successful Programming and/or Debugging:

• PIN4 - RA5/MCLR/VPP - for RESET and Programming voltage (RESET required for PIC to enter Programming mode, etc.)
• PIN9 - RB3/PGM/CCP1(x) - for Lov Voltage Programming (LVP) mode
• PIN12 - RB6/PGC/T1OSO/T1CKI - Programmer/debugger Clock
• PIN13 - RB7/PGD/T1OSI - Programmer/Debugger Data

If you connect load or capacity on any of above pins it may interfere with Programming and/or Debugging this chip.

PWM math

Have to combine data from:

• PIC16F88, page 84, section 9.3 PWM Mode
• DS40001369C, page 98, section 9.2.2 PWM Period
• DS31014A, page 210 (14-8), section 14.5. PWM Mode

According to PIC16F88 datasheet, page 84, section 9.3.1 PWM Period there is:

$$PWM_{per} = \frac{ ( PR2 + 1 ) \ast 4 \ast TMR2 }{ F_{osc} }$$

So

$$PR2 + 1 = \frac{ PWM_{per} \ast F_{osc} }{ 4 \ast TMR2_{presc} }$$

And finally:

$$PR2 = \frac{ PWM_{per} \ast F_{osc} }{ 4 \ast TMR2_{presc} } - 1$$

Or

$$PR2 = \frac{ F_{osc} }{ 4 \ast TMR2_{presc} \ast PWM_{freq} } - 1$$

Example for expected $PWM_{freq} = 1000$ Hz)

$$PR2 = \frac{ 4 000 000 }{ 4 \ast 1 \ast 1 000 } - 1 = \frac{ 4 000 000 }{ 4 000 } - 1 = 99$$

So $PR2$ should be $99$.

Now we need to compute value Pulse Width 10-bit register CCPR. Let's say we want Pulse Width 25% = Ratio 0.25 (ratio is from 0 to 0.99999). The formula is:

$$DutyRatio = \frac{ CCPR_{10bit} }{ 4 \ast ( PR2 + 1 ) }$$

Thus

$$CCPR_{10bit} = DutyRatio \ast 4 \ast (PR2 + 1 ) = DutyRatio \ast 400$$

For ratio 25% => 0.25 we get $CCPR_{10bit} = 0.25 \ast 400 = 100 $

C Version

Project under pic16f88_adc_pwm_c.X/. Requires latest software:

• MPLAB X IDE v6.05
• XC8 v2.40
• DFP 1.3.42

Listing is generated by default (at least under MPLAB X IDE v6.50):

pic16f88_adc_pwm_c.X/dist/default/production/pic16f88_adc_pwm_c.X.production.lst

• thanks to Project Properties -> XC8 Compiler -> Preprocessing and messages -> Generate ASM Listing file

Linker map file is generated in:

pic16f88_adc_pwm_c.X/dist/default/production/pic16f88_adc_pwm_c.X.production.map

Resources

• used my PIC16F88 project as template:
  • https://github.com/hpaluch/pic16f88-comp-ccp-ex.X
• PICDEM Lab Development Board User Guide
  • DS40001369C
  • used above guide for modified ADC circuit and ADC and PWM setup verification - but please note that this guide uses different PIC

Footnotes

1. PIC16F88 is one of few that has In-Circuit Debugger (ICD) which means that you can use them directly with suitable Debugger (PicKit3, ICD3, ICD4,...). Many other 8-bit PICs can't be debugged without special Adapter with emulation chip (called AC header). ↩