PROPORTIONAL PRESCALER

[carl]

Please could anybody help me with or suggest the best solution to a counting/timer problem I have.

Using: PIC18F4525
xtal: 4MHz

what I need to do is to count up to 65535, over a variable time period, from 5 seconds to 300 seconds.

so clock pulse = 4MHz/4 = 1us
and; 65536 x 1us = 65.536ms (minimum/fastest time taken for 65536 counts)

now I will have a variable 't' equating to (5s-300s). this value must be used in some easy way to set the prescaler to count at this speed/time.

e.g for 10s:

10s/65.536ms = gain of 152.58
and 1us x 152.58 = 152.58us.


therefore the prescaler would have to be configured to give an output every 152.58us.

and my question is, is there such a way to set the prescaler for a variable which has nearly 300 possibilities - or am I going up the wrong track.
Regards
Carl

[PCM programmer]

[carl]

thanks for replying PCM Programmer,

I want to output(on 2 ports) the value 's' from 0 to 65535 - this is then converted into a voltage later on with a D/A converter. BUT THIS IS DONE OVER A VARIABLE TIME PERIOD.

There is no input, just the xtal. So i need to manipulate and increment the time period/counters/timers/prescalers. from this I can then do one of two options (I think?):

1) for every count change, output (write) a proportionally timed signal to one pin. then read this pin and for every h_L transition - increment 's', and for every L_H transition, write 's' to the 2 ports.

2)just do it in software with timers so that for every count change the value is written to the 2 ports.

[PCM programmer]

What is the overall purpose of doing this ?

[carl]

this is being used for a 4_20mA Loop Calibrator.
particular modes of these devices work by ramping up and down from 4_20mA and back again OVER A PARTICULAR TIME PERIOD.

therefore the starting point is to get the output from the pic to write the 16bit value over a variable time period.

THE TIME PERIOD IS SET BY THE USER ANYWHERE FROM 5s to 300s.

[Humberto]

[libor]

What do you, guys, use for 4-20mA output ? I was looking for schematics but only could find solutions with custom ICs like the XTR-1xx series. Do you use them or do you have your own circuits ?

[carl]

Humberto,

This is exactly what i am after. I am designing a 16 bit 4_20mA converter with variable ramp time up/down times. I know that the calibrators on the market at present only have a few standard ramp times - that is why I want a variable one. You say it can be done without changing the prescaler - I would be very much in your debt if you could help.

Libor,
The best idea is to use a dedicated IC for the job like your xtr115 or xtr110 - I am using something different because of availability problems. Bear in mind that there are two types - those that simulate and those that source. the difference being that one needs an external power supply in the loop and the other one doesn't. also please bear in mind that it is the variable input (voltage or current) going into the transmitter that is proportional to the 4_20mA output.

[RLScott]

[Humberto]

libor wrote:

[carl]

Dear RL Scott, Thankyou so much for your response. I read quite a bit on your method and lots of replies stated that this was not a good way to use an interrupt because the counter is still counting whilst the ISR is being serviced. Although it looks as though I am going to have to go down this route because setting the prescaler is limited - (not enough divide's by to make it accurate).

As for the steps they are in 1 second increments so the accuracy should be able to be done as you stated. I did not state all the parameters needed when I first posted this question because I did not want to baffle anyone but the 76us would actually be 38us because I need to count upto 65535 and then back down again within the time period - 36us per step up and 36us per step down. THIS IS FOR THE 5s CHOICE.

Also I need to poll the value every 10ms and display on lcd. Do you think I have enough time to play with to perform all these tasks - or if not I suppose getting a fatser clock would help.

This is all secondary to the fact that I am new to C and the above seems tricky. I am getting the hang of it but find examples and books really usefull sources - any examples of the above sistuation?

Your response was very much appreciated - thankyou Robert Scott and Humberto

[RLScott]

[carl]

Thanks Robert for getting back,
Yep I knew the 10ms refresh was rubbish - made an error there - I will make it 500ms or something like that.

[RLScott]

[carl]

Hi Robert,

I'm going to get a faster xtal and give it a go. First I will absorb everything already discussed and learn how to use ISR's. Thanks for your help and I will keep you updated.

Have you got any good examples to look at?
Carl