Need to PWM 4 outs

T Chap

Update: I am moving this to Propeller as it is probably better suited and may be simpler to put all code under one roof.



I am rebuilding a CNC machine controller using the existing PS, connectors, box, etc. All I am doing is replacing the PIC16F84A with an SX28. The motors are unipolar 6 wire, so I need to turn on and off 4 transistors that are already on the board(TIP120). The board does not have sense resistors, they are PWMing at a fixed approx 50duty cycle at .03125 microsecond frequency. I could do either: leave the pins high unless output is active, in that case pulse low for each PWM cycle, or the opposite.

The inputs from the PC are step and direction, and can be altered to active low for each. I need to sequence each step input based on the dir pin. I am not sure the best way to do this. If you are moving the motor forward, then you want to change Dir, do you go to the previous Sequence, or start over at sequence A and then sequence in reverse?

Here is just a crude start to simulate PWM output. I am pretty much stuck on how to do this code so that it will toggle in each new sequence according to the direction. Also, I really need this to half step, in which case I need half stepping sequences, but I figured I need to start simple.

TIP1 VAR RA.0 'coil 1
TIP2 VAR RA.1 '2
TIP3 VAR RA.2 '3
TIP4 VAR RA.3 '4

Step VAR RB.0
DR VAR RB.1

Seq0 = %0000
SeqA = %0101
SeqB = %0110
SeqC = %1010
SeqD = %1001


'setup

TIP1 = 1
TIP2 = 1
TIP3 = 1
TIP4 = 1


MAIN:

IF STEP = 1 AND DR = 1 then? 'advance one sequence
IF STEP = 1 AND DR = 0 then ? 'back up one sequence

GOTO main

SequenceA:
RA = SeqA
'pause .015 micro seconds
RA = Seq0
'pause .015 micro seconds

'put new IF thens here for New STEP pulse?

GOTO SequenceA 'how to loop here, but get back to main if new STEP?

SequenceB:
RA = SeqB
'pause .015 micro seconds
RA = Seq0
'pause .015 micro seconds
GOTO SequenceB

SequenceC:
RA = SeqC
'pause .015 micro seconds
RA = Seq0
'pause .015 micro seconds
GOTO SequenceC

SequenceD:
RA = SeqD
'pause .015 micro seconds
RA = Seq0
'pause .015 micro seconds
GOTO SequenceD


Thanks for any suggestions how to proceed.

Post Edited (originator99) : 9/29/2006 11:25:54 PM GMT

pjv

Hi Originator;

Do you really mean 15 nano seconds (0.015 uSec).... I suspect what you want is much longer!

The sequences and directions are easy when you employ a state machine configuration. On top of that, you can simply turn off the selected transistors by ANDing the port bits with 0000 for the desired duration.

This is all quite possible using interrupts and a simple-minded real-time scheduler.

Cheers,

Peter (pjv)

T Chap

no, .015 microseconds to accomplish what I had with the old PIC. On my scope it was at .5 microsecond sampling, and had 16 cycles on the screen, so I derived .5 micro / 16 = .03125 microsecond PWM freq.

Bean

I think you might be measuring something wrong.
16 cycles in 0.5uSec = 32 MHz ! I can't believe the pwm is that fast from a PIC.

Bean.

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T Chap

Well I looked at it again. It just depends on how you are looking at it what it really is on my junk scope. On the one hand, you have 8 cycles in one 50 uS division. But if you really zoom in further, you see a fixed clock rate dividing up the cycles just mentioned. Those tiny divisions of the larger divisions make up the small figure I said which shoudl have been stated at 16 cycles divided by .4uS division.

Post Edited (originator99) : 9/30/2006 5:08:53 AM GMT