Voltage output from PIC16f628A

[bennamanna]

Hi all,

I am very new to pics and programming. I have a PIC16f628A of which I want to use for a project. I wish to have 2 voltage outputs. One 0-2.5v and the other 0-5v. I wish to be able to to send commands from a PC via rs232 to set these voltages. I am using picbasic for the project. Can anyone give me some guidance or some code of a similar project that I can learn from? The pc coding is fine. I am using VB and comm control.

Thanks,

Ben

[mister_e]

Tons of different way

Scheme 1:
Use HPWM to produce your 0-5 volt
Use a op-amp and use the Vref signal to produce you 2nd 0-2.5 volt. If you can deal with the resolution.. it works.

Scheme2:
Use HPWM for your 0-5 volt
Create a timer based PWM to create the second PWM channel

Scheme3:
Use 2 DAC. MAX437 is simple and cheap

Scheme4:
Use Darrel Taylor Multi SSPWM. So you may produce few different voltage output.

Scheme5:
Create you own multi PWM output based on Timer interrupts.

Scheme6:
Use a PIC with 2 CCP module.. hence two PWM output. 16F876,16F877 and the list is long enough to fill the screen here

Scheme7:
Use 2 PIC with 1 PWM. 1XPIC16F628 + PIC 12F683 talking together.

There's some others but it should be enough to start?

[bennamanna]

Gday,

Thanks for your reply. I am testing some of your methods. I have a few queries.

1. This voltage i require is to be read by a vehicle ecu. If you use PWM will it see that as a voltage or will it see the pulses?

2. I have put together some code to test the PWM output. It is giving a PWM signal as expected but when I input a figure in a terminal it does not put the same figure to the display or PWM output. It does change but not as expected. Any ideas why?

Thanks


INCLUDE "modedefs.bas" ' Include serial modes
SI VAR PORTB.1 ' Serial Input pin
B0 VAR BYTE
Define OSC 10

Define LCD_DREG PORTA ' RA0..3
Define LCD_DBIT 0
Define LCD_RSREG PORTA ' RA6
Define LCD_RSBIT 6
Define LCD_EREG PORTA ' RA4
Define LCD_EBIT 4
Define LCD_BITS 4
TRISA = 0
TRISB = 0
CMCON = 7

Pause 1000 ' Wait for LCD to startup



duty VAR WORD ' Duty cycle value (CCPR3L:CCP3CON<5:4>)


TRISB.3 = 0 ' Set PORTB.5 (CCP3) to output
CCP1CON = %00001100 ' Set CCP3 to PWM
T2CON = %00010101 ' Turn on Timer2, Prescale=4

' Use formula to determine PR2 value for a 1KHz signal,
' 4MHz clock, and prescale=4. (4E6/(4*4*1E3))-1=249

PR2 = 249 ' Set PR2 to get 1KHz out

' Use formula to determine CCPR3L:CCP3CON<5:4> value for
' ends of range 20% to 80%. (249+1)*4*0.2=200 (20% value)
' (249+1)*4*0.8=800 (80% value)


duty = 200 ' Set duty cycle to 20%

loop: SerIn SI,T9600,duty
CCP1CON.4 = duty.0 ' Store duty to registers as
CCP1CON.5 = duty.1 ' a 10-bit word
CCPR1L = duty >> 2

'duty = duty + 5 ' Increase duty cycle
LCDOut $FE,1 ' Clear LCD screen
LCDOut # duty ' Display line 1
pause 1000
' Since the total sweep of duty is 600 (800-200) and
' we are adding 10 for each loop, that results in 60
' steps min to max. 1 second divided by 60 = 16.67mS

Pause 17 ' Pause 1/60 of second

'IF (duty < 900) Then loop ' Do it again unless 80% duty cycle

'duty = 200 ' Reset to 20% duty cycle

GoTo loop ' Do it forever

[mister_e]

your CCP definition are fine for 4MHZ, but not for 10MHZ.

download this tool bellow
http://www.picbasic.co.uk/forum/showthread.php?t=4994

it will give you some answer.

1. This voltage i require is to be read by a vehicle ecu. If you use PWM will it see that as a voltage or will it see the pulses?

More than likely it will see pulses. You have to convert it to pure DC with a simple RC filter... better if you add a op-amp buffer in between.

[bennamanna]

Gday,

Thanks again for your reply. I will use a rc filter as you suggested.

I have downloaded your calc. It looks like it would be verry usefull. As stated I am a beginner ;- ( Could you please step me through what I have to enter in to your calc and my code to fix it? I will learn this way.

Thanks,

Ben

[mister_e]

you just need to enter the OSC speed and expected frequency. it will give you...
<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=1168&stc=1&d=116295770 1">
There, you have the
prescaller => 1:16 => see T2CON setting in your datasheet
PR2 => 155 => directly

It also give you the value for 100% duty cycle => 625

If you want to set different DutyCycle, move the scroll bar at the top right. The duty value will be reported in the 'results' section at the bottom.

Not much.