Selecting the A/D Conversion

Guest · Posted: Fri Apr 09, 2004 1:27 am

Hello
On A/D of PIC16F877 the datasheet says The A/D conversion time per bit is defined as TAD. The A/D conversion requires a minimum 12TAD per 10-bit conversion. It also says for crystal frequencies up to 5Mhz, TAD should be 8TOSC, and for crystal frequencies up to 20MHz, it should be set to 32TOSC. Now let's do some calculations:

For a 4MHz crystal:
TAD=8TOSC=2us
Time needed to finish one conversion=12TAD=24us

For a 8MHz crystal:
TAD=32TOSC=4us
Time needed to finish one conversion=12TAD=48us (!)

So does this mean doubling the crystal frequency will effectively slow down the A/D conversion by a factor of two?

Guest · Posted: Fri Apr 09, 2004 1:38 am

"For correct A/D conversions, the A/D conversion clock
(TAD) must be selected to ensure a minimum TAD time
of 1.6 us."
Using 4MHz and div8 you can fit this req. but with 8MHz you have to use div32 (div8 gives 1usec TAD what is out of range).
Try to use the internal RC osc.

Guest · Posted: Fri Apr 09, 2004 4:20 am

Sorry, but that was not the answer I was looking for. I had a,ready mentioned I needed to use 32TOSC.
And the datasheet doesn not recommend using internal RC for high crystal frequencies such as 8MHz.
Anybody can help me here please?

Ttelmah · Guest

Guest · Posted: Fri Apr 09, 2004 6:08 am

Thanks. So does this mean if I double my XTAL from 4MHz to 8MHz, it might actually slow down my circuit (depending on the code, because of the A/D conversion time bottleneck)?

Guest · Posted: Fri Apr 09, 2004 11:11 am

8/4 < 32/8 is your question? I am pretty sure that 2<4. Especially in this season...