Pulsing an output under hardware timer control

[pjrebordao]

Using a 28X1, is there a way to set an output pin pulsing in the background, without having to go through an :interrupt service routine ? Maybe by directly setting some hardware registers ?

I need pulses every 1ms - 1.5ms

 

[hippy]

You could use PWMOUT. That normally only works down to around 4kHz but by poking prescaler SFR's people have had it running slower so around 1kHz should be achievable.

Have a search on the PICAXE forum for previous discussion ( as PWM is less than four letters, use PWM* as a search term ).

An alternative is to use another smaller PICAXE ( 08 perhaps ) and use that to generate the pulsing, enabled and disabled by the 28X1, and you may even be able to make it programmable by the28X1 to set a specific pulsing rate.

 

[pjrebordao]

I'll have a look at the SFR possibilities. The 08M doesn't seem an option if I understand you , as I need more accuracy. I'm using a 20Mhz ressonator with the 28X1.

 

[hippy]

The 08 is entirely capable of generating a pulse every 1ms to 1.5ms. Are there additional requirements which you have not stated ?

 

[eclectic]

Paulo.

What is the duration of the pulse?

What is the frequency?

What is the accuracy required?

e

 

[pjrebordao]

Frequency: 1-1.5 Khz
Duration: approx 50% duty cycle
Accuracy / Stability: The one you can get from a quartz oscillator (1% ?)

 

[eclectic]

Paulo.

Just some ideas for you to test, using your 'scope.

#picaxe 28X1
setfreq m4
symbol duty = w1
main:


For b0 = 166 to 249

duty = b0 *2 +1

pwmout 2,b0,duty
; ~6 kHz - 4 kHz "normal" pwmout

pause 5

poke $12,5
; freq / 4 

pause 2000
next
goto main

Original idea from

http://www.picaxeforum.co.uk/showthread.php?t=5872&highlight=jeremy+PWMOUT

e

 

[pjrebordao]

Haven't tried yet, just looked at code...
But how does the 5 (as in poke $12, 5) translates into 4 (as in freq/4) ?

 

[eclectic]

Haven't tried yet, just looked at code...
But how does the 5 (as in poke $12, 5) translates into 4 (as in freq/4) ?

Run Dippy's program on a real chip
and look at the Sertxd values.

e

 

[hippy]

Frequency: 1-1.5 Khz

That's a pulse rate of 6.66ms to 1ms, not the earlier 1ms to 1.5ms.

Are you saying you want to vary the pulse rate frequency between 1kHz and 1.5kHz ( by what amount of stepping ) or just want to produce a single rate between those frequencies ?

Accuracy / Stability: The one you can get from a quartz oscillator (1% ?)

About 0.003% with a 30ppm accurate crystal. Do you really need that accuracy or is 1% acceptable ?

 

[pjrebordao]

The purpose of this frequency is to "drive" a stepper driver IC for a turntable that I'm building / upgrading. As such, accuracy (as from a crystal) is paramount, or else I'll get some warbly tunes...

Anyway, I'm now confident that by changing the transmission ratio to the platter and changing the clock for the picaxe, I can get pwmout (by poking $12) achieve the rates I need. I'll need two different ones, for 33rpm and 45rpm records.

 

[BeanieBots]

The 'rumble' you will get from using a stepper motor will be orders of magnitude more noticeable than any "wow & flutter" you'll get from frequency drift even using a homemade RC oscillator!

 

[pjrebordao]

Actually no. The stepper is driven by microstepping thus "approximating" a sinewave and greatly reducing vibrations.
I'm just getting some small w&f because the belt tension isn't right. I forgot to add the means to adjust it !