PWM on PORTG pin 3 on a 18F67k22

Hi guys ,

i am trying the get PWM (CCP4) output to work on PortG.3 of the 18F67k22 series

what i am seeing so far is that the port seems to put out some pulses then stops , so it seems that i have part of the setup correct

i am thinking that i am also effecting other ports with the setting as other activity on portF , and G seem to sometimes have an effect , but i cant seem to isolate exactly what it is so far

the output pulses rates etc dont matter at this point just as long as it outputs all the time for a test of the PWM function on this pin only

what am i missing


cheers

sheldon


http://ww1.microchip.com/downloads/e...Doc/39960d.pdf

Code:

---

'    -------  Setup port G Variables  ----------   
  ' port G has 6 I/O only , G5 - Always input ( uses CONFIG3H - MCLRE = OFF for I/O use) 
    CM3CON = 0                ' Disable comparator 3 on PORTG
   
    ANCON2.0 = 0              ' Set PortG:4 Analog / Digital allocation - 0 = digital ,1 analog - ANSEL16
    ANCON2.1 = 0              ' Set PortG:3 Analog / Digital allocation - 0 = digital ,1 analog - ANSEL17
    ANCON2.2 = 0              ' Set PortG:2 Analog / Digital allocation - 0 = digital ,1 analog - ANSEL18
    ANCON2.3 = 0              ' Set PortG:1 Analog / Digital allocation - 0 = digital ,1 analog - ANSEL19
   
    TRISG = %00100000          ' ports Output , Row_1 output  portG.5 always input ( internal pullup) VPP/MCLR ,CCP4 output
    FLASH_CE    var PORTG.2  ' FLASH CE - active low - uses SPI bus
    PWM_TEST VAR PORTG.3  ' CCP4 - PWM outout to control the contrast on glcd     
 
 
  PORTG  =  %00100000        ' Clear Outputs ,
  flash_ce = 1              ' set high as active low
  PWM_TEST = 0          ' active high

---

Code:

---


'---------- PWM OUTPUT REGISTER SETUP ---------------------
    ' setup CCP4 - PORTG.3 for PWM_CONTRAST control
    ' PWM FRQ = 1/PWM period
    ' When TMR2 = PR2 = start of duty cycle(on Time ) , start of period length
    ' when TMR2 = CCPR4L:CCP4CON5:4 = End of duty cycle , START OF OFF TIME 
    ' when TMR2 = PR2 = End of period , start of duty cycle AGAIN
 
 
    CCPTMRS1 = %00000000  ' select TM1/TM2 for CCP4 use ( TIMER2 used for PWM)                     
    CCP4CON  = %00001100  ' Setup CCP4 single output for PWM , bits 3,2 , bits 5,4 = 9th and 10 bit of duty cycle ,
    CCPR4L  = 100        ' Lower Byte register for DUTY CYCLE OF PWM  - 8 bits of duty cycle
    ODCON2.1  = 0          ' set CCP4OD pin not enabled FOR OPen drain 
    PR2      = 100        ' set the PWM period = [(PR2+1)x 4xTosc / TM2 prescale value 
    T2CON    = %00000101  ' Timer 2 ON - POSTSCALLER SETTING NOT USED BY PWM , OLY PRESCALE SETTING - PRESCALE: 00 =1 ,01 = 4,1x = 16
                          ' ( bit 7 = x, bit6-3 postscale, bit2=0 timer off ,bit1-0 = prescale )
    TMR2    =  200

---

This is a HPWM pin?
Why not use HPWM you must have your reasons I guess.

steve this is the first time i need a hardware based pwm , so dont assume i got it right

i did use darrel software based one before , but i need this pwm to be set and forget , and not use up cpu time if it were based as software

and from what i read HPWM command has a speed limitations and the settings are often best set in via the registers directly

hpwm is set and forget
the only limitations are max freq = 32767hz min freq depends on fosc , and the pwm resolution is 8 bits , should be adequate for lcd contrast control.

in your example depending on fosc the tmr2 prescale value is way to low try 16 the pwm resolution might only be about 4 bits (read the data sheet)

i think i found my mistake steve, a solder bridge on the output to another pin that was being set , which caused an issue when the PWM was output

d

ok thanks richard i am tryng the to get a contrast control working for neg voltage , came across this example , see attached

i still want to have a trim pot , which will set the default value so the lcd is viewable regardless , as well a setting it up via software , to get better viewable / allow temperature change

have you come across other working examples for negative bios contrast controls

I use something like this , note drive fet pin high at reset and gradually decrease the pwm till the contrast is good

that example looks like the display contrast control only swing between vdd and vss , not a negative voltage of vee ???

its actually a switch mode power supply that creates the 0-12v contrast voltage , the output voltage is inversely proportional to the pwm

http://www.digikey.com/en/articles/t...-input-voltage

a better link
http://www.jaycar.com.au/images_uploaded/dcdcconv.pdf
see fig 3

thanks richard , the second link helped clear why it works ,

i happened to have LM324 onhand and the first cct seems to work well , changing the r4, r9 in the diagram to 200k pot allows easy way to set the best display level as a dephalt setting for the display that is in use for testing , and will go back to a set resistors now that it has been found

the display i am using has a -9.5v VEE generated output , seems best display with -7v ( relative to GND) applied to to V0

for anyone using this cct the OP amp series LM321( single) LM358(duel),LM324(quad) , has a voltage range of upto 32v ( combined 5v + VEE = 32v( VEE upto -27v))

ill do further testing , but i found the best results to be for a freq of 1300 , which gave a smooth voltage output from the op amp , a duty of 140 , ranging from 80 - 200 gave a voltage shift of +/- 2V from pot setting

in the end it was alot easier to use the HPWM command , as the freq required was well below 32767Hz no direct chip register setting up was required

cheers guys