Read analog sensors in the background?

[MO]

Hallo,

I want to read 6 analog sensors and update their accosiated globals. I tried using int_ad with the next a/d conversion started at the end of the ad_isr, but this kept my program almost exclusively in the ad_isr leaving very little time for main() and other routines.

I'm using a PIC18F4620 with 20MHz crystal. I use setup_adc_ports (AN0_TO_AN7|VSS_VDD) with setup_adc(ADC_CLOCK_DIV_32). I am already using int_rda, int_ext1, int_rb and timer0(20ms timebase), timer2(pwm) and timer3(software pwm).

I want to update the analog readings so that my main() and other routines can use the most current readings without having to query and wait for the a/d convertion to finish.

What is a good strategy for doing this? Please point me in an efficient direction.

Thanks for the guidance!

Regards,
Madri

[Ttelmah]

Have a timer 'tick' interrupt. Interval to suit you, but typically perhaps about 50 to 100Hz.
Then have this use a state machine, something like:

[Guest]

Thank Ttelmah for the quick reply - and the code too! This is fantastic - just what I was hoping to be able to do and not able to get my mind around how to go about it.

It works great!

Thank you so much for this - made my day for sure. Maybe even week.
Madri

[asmboy]

i have been doing almost the identical thing for over a year with a four analog channel rotation, but using just 2 states :


state ZERO, TWO and THREE can be combined very efficiently
by the sequence:

STATE: 0
Start ADC CHAN
state =!state;

STATE:1
1- READ_ADC,
2- adjust chan number glo var ,
3- set next ADC_CHAN ( for acquisition time) -
state =!state;


then the reading is STARTED on the next tick,
the OTHER state of the machine )


there are only two time consuming things being done ( for good reading accuracy) and they are (1) settling time - which occurs after the chan is switched "set adc chan" and (2) conversion time "start adc conv"

this 2 state machine allocates ( equal enough ) time to each

the reason to go this way ?

Your acq rate is doubled over ttelmahs nice example and
the extra time lingering in the ISR is diddly , since other than ADC START and the ACQ time delay after chan set - all instrux are wholly deterministic.

this pretty evenly splits ACQ time and AD/CONVERT time between alternating 'ticks'

you do need to INIT the process vars of course before first call

in my implementation i actually use a rolling average integrator that sums 4 sets of readings on each of four channels and updates the running average every 16 tix - of 8.19ms , on an ~20 mhz system but that is another story

[Ttelmah]

Yes. In fact my normal one does the 'select next channel', immediately after reading the value, reducing the states by one. I have posted versions of this in the past.
However I normally keep one more state than the minimum, since I typically perform some averaging functionality, and prefer to perform the maths in an extra state. You can keep the arithmetic very fast, if you only perform integer addition, and integer division by a binary factor (2,4,8), using the >> function, avoiding maths re-entancy problems, and still keeping the ticks fast.

It is worth also realising, that if you have the tick at some nice division of a second, using perhaps timer2, you can also do a lot of your basic system timings, with something like: