My first foray into PWM

[Demon]

Before I get too deep in this, can I blink an LED ON for 500uS, OFF for 500uS using standard PWM module?

Or am I trying to use a Ferrari as a baby stroller?

I know I can use DT_INT but I know how to do that and want to avoid using cycles. The PWM hardware is there doing nothing and I was hoping I could use it as a heartbeat during my tests.

I've printed pages 186-189 of the 18F44K22 datasheet and it doesn't seem overwhelming at first glance. The only part I can't determine by reading that is if I can use it as slow as I want. I know this is meant to drive motors so it's entirely possible that it can't physically go this slow.

Robert


EDIT: I'm running at 64MHz so I can guarantee persistence of vision while blinking at least 50 LEDs, manage a keypad and handle USART comms (at the very least). But I'm getting a feeling by looking at that PWM PERIOD formula at 14-1 that I could only do this if I slow to a crawl.

[richard]

if you are using the hpwm command the max freq is 32767hz the min @64mhz ck is 3905 hz on a pic18
max period 256uS min 30.5uS
so you could at 50% duty get 128uS on 128uS off

[Demon]

Yes, I definitely want the hardware PWM. But it's not the 50% duty that I was concerned with, it was the 1 second period.

I see now the relation between clock speed and the PWM output. So there's no way I can get it slow enough to use it as a heartbeat (1 sec period) in the background if we limitted with a 4MHz lower range.

I was hoping to use it as a debugging tool; to show that a PIC is indeed working without taking any clock cycles (disturbing cycle sensitive logic). And I wanted to try out HPWM, I'm nowhere near ready to get back on my CNC project; have to complete my residential alarm system first (and I'd like to get that interfacing with android, text messenging and web page - lots of learning).

Robert

[Amoque]

You might, if you have one laying around, use a clock divider chip... or a counter... Here's a link that explains my thinking better than I can.

[richard]

You might be able to achieve something using compare mode and the external clock pin for timer1. if you have enough spare pins and if you were really clever
use the hpwm output to drive tmr1ck then you could change the flash rate on the fly

[Demon]

Euh, I think I'll just get back to finishing my Charlie-plexing work for now. LOL

Thanks though, I will have to jump into PWM one day, hopefully next summer.

Robert

[HenrikOlsson]

Hi Robert,
In your first post you say LED ON for 500uS, OFF for 500uS but then you say you want a period of 1s - that's a difference of $03E8 times.
I know you didn't want to do it with an normal interrupts but I'm going to suggest a simple timer interrupt anyway.... At such low frequencies (even though you're probably not going to get 500ms interrupt period in "on go") the interrupt processing will have very little impact on the overall performance.

/Henrik.

[gadelhas]

Hi Robert;

An HeartBeat Led should be do it with a timer interrupt like Henrik sujested....
If you cannot achieve tht time that you want, you do for instance, a 100ms interrupt and with a counter you can achieve the time that you want counting in 100ms spaces.
You can count 5 x 100ms and then toogle the led.

[Demon]

Hi Robert,
In your first post you say LED ON for 500uS, OFF for 500uS but then you say you want a period of 1s - that's a difference of $03E8 times.
...

(without considering oscillator speed and PWM range limitations)

Then there's something about PWM that I didn't understand. HIGH/LOW using PAUSE for 500uS is not the same as a 1000uS period with 50% duty cycle?


Ooop, uS, not Us.

Robert

[gadelhas]

(without considering oscillator speed and PWM range limitations)

Then there's something about PWM that I didn't understand. HIGH/LOW using PAUSE for 500uS is not the same as a 1000uS period with 50% duty cycle?

Attachment 7531
Ooop, uS, not Us.

Robert

Yes, from that point of view, it is the same thing! (without considering oscillator speed and PWM range limitations)

[HenrikOlsson]

Then there's something about PWM that I didn't understand. HIGH/LOW using PAUSE for 500uS is not the same as a 1000uS period with 50% duty cycle?

Yes of course, 500us ON and 500us OFF is 50% dutycyle. But the period is 500us+500us = 1000us = 1ms = 0.001s - NOT 1s which you later wrote.

In your first post you indicated you wanted a PWM period of 1000us (a frequency of 1000Hz). In your second post you said you wanted a period of 1s (a frequency of 1Hz). It doesn't add up - that's all I was trying to say.

/Henrik.

[Demon]

Ooops, my bad. Didn't explain myself properly.

[richard]

how to flash a pulse led @ 1 hz on a pic18 @64mhz with no code overhead and no interrupts

for demon

'*  Name    : UNTITLED.BAS                                      *
'*  Author  : [select VIEW...EDITOR OPTIONS]                    *
'*  Notice  : Copyright (c) 2014 [select VIEW...EDITOR OPTIONS] *
'*          : All Rights Reserved                               *
'*  Date    : 29/11/2014                                        *
'*  Version : 1.0                                               *
'*  Notes   :   pic18f45k20                                                *
'*          :                                                 *
'****************************************************************
#CONFIG
  CONFIG  FOSC = INTIO67
  CONFIG  FCMEN = OFF
  CONFIG  IESO = OFF
  CONFIG  PWRT = OFF
  CONFIG  BOREN = SBORDIS
  CONFIG  BORV = 18
  CONFIG  WDTEN = ON
  CONFIG  WDTPS = 512
  CONFIG  CCP2MX = PORTC
  CONFIG  PBADEN = OFF
  CONFIG  LPT1OSC = OFF
  CONFIG  HFOFST = ON
  CONFIG  MCLRE = ON
  CONFIG  STVREN = ON
  CONFIG  LVP = OFF
  CONFIG  XINST = OFF
  CONFIG  DEBUG = OFF
  CONFIG  CP0 = OFF
  CONFIG  CP1 = OFF
  CONFIG  CP2 = OFF
  CONFIG  CP3 = OFF
  CONFIG  CPB = OFF
  CONFIG  CPD = OFF
  CONFIG  WRT0 = OFF
  CONFIG  WRT1 = OFF
  CONFIG  WRT2 = OFF
  CONFIG  WRT3 = OFF
  CONFIG  WRTC = OFF
  CONFIG  WRTB = OFF
  CONFIG  WRTD = OFF
  CONFIG  EBTR0 = OFF
  CONFIG  EBTR1 = OFF
  CONFIG  EBTR2 = OFF
  CONFIG  EBTR3 = OFF
  CONFIG  EBTRB = OFF
#ENDCONFIG

  define OSC 64
  
   osccon=$70   '4 mhz
    
     trisc=%11111001
  
   trisd.1=0   ' another led 
     T2CON = $4; // tmr2 prescale 0 and on
     PR2 = 255; // pwm period   ?
     CCP1CON = $c; // pwm mode
     CCPR1L = 127; // pw
     t1con=$43
     CCP2CON = 2
     t3con=3
     OSCTUNE.6=1 

   ; ccp1 out connects to t1ck in and pulse led connected to ccp2 out
   ; on     pic18f45k20  ccp1 out = portc.2
   ;                     ccp2 out = portc.1  
   ;                     ti ck in = portc.0

 

 
  
  
  
    
Main:
  latd.1= !latd.1       ;flash another led
  pause 500
  goto main

[Demon]

Neat, first I have to get my Charlie-plex going again. I broke it. LOL

Robert