Fast / low resolution A/D conversion

[AndrewC]

I'm considering an application which requires a 12F683 to perform a very fast A/D conversion - I don't need the full 10 bits the ADC normally supplies, probably just 4 bits of resolution is adequate. Reading the data sheet it says that if you terminate the conversion early (after say 5 TAD periods) nothing is stored. Is there a trick to get around this ?

Andrew

[mister_e]

How fast is enough? Do you detect some specific range/values, or just a threshold/single value?

[Acetronics2]

Hi, Andrew

Such use of ADCs are explained in the PIC Datasheets relevant chapters and Midrange Manual.

Good explanation also from the great " Bigonoff " ( a good asm teacher from Belgium ) - in French !

see Part 2 $19.6 ...

Alain

[AndrewC]

Hi, Andrew

Such use of ADCs are explained in the PIC Datasheets relevant chapters and Midrange Manual.

Alain

I couldn't find where to reduce the ADC resolution - can you be more explicit ?

Andrew

[Acetronics2]

You didn't search too much ...

23.7.1 Faster Conversion - Lower Resolution Trade-off

Alain

[AndrewC]

Or perhaps not looking in the right place. It doesn't refer to it in the device datasheet but does in the midrange manual.

It's a pity you can't just stop the conversion after a certain number of bits.

Anyway, thanks for pointing me in the right direction.

Andrew

[AndrewC]

How fast is enough? Do you detect some specific range/values, or just a threshold/single value?

Application is to detect a change in light intensity caused by an object passing in front of a confocally illuminated machine vision lens. It is battery powered so I want a low duty cycle illumination but fast enough to detect moving subjects.

What i'm thinking is a cycle that runs:

1) Light on
2) Pause enough for sampling cap to charge
3) Start AD conversion
4) Light off
5) Interrupt on conversion complete and compare new intensity to old - GOTO event handler if significant change
6) Pause and loop

What I want is a total loop time of <1msec or less. I can run a TAD of 2usec so it would only be 22usec for the conversion which to be honest is probably way fast enough. It just seems wasteful to do a 10bit conversion when I only want 4 bits of data !

Andrew