Hi All,

Thanks in advance for any help you can give me with a problem I am having:

I am trying to use the ADC on a 16C71 and I am getting an error of as much
as 2 LSB (when the datasheet says +/- 1 LSB max). I am measuring the
voltage directly between the ADC pin and the PIC's Vss pin and the output
code seems to be off. I am waiting about 800 uS from the time I enable the
ADC until I begin the conversion, and the output impedance of my source is
2.2K

Now as to the reason why this is entitled "SLEEP": AS recommended in the
datasheet, I am placing the pic in sleep mode during the ADC,using the
internal RC oscillator for the ADC. My normal oscillator element is a 4MHz
xtal with 20 pF load caps. When I observe the pic's oscillator pins with my
scope (about 20pF loading with my x10 probe),instead of seeing the
oscillator exponentially decay to zero during sleep (as I would expect), I
see the DC level increase slightly and the the amplitude remain constant.
This region of the waveform is about 50 uS long after which there is a
small glitch and then the DC level returns back to the same as before
sleep. Is this normal for sleep mode? I thought the whole idea was to
disable the oscillator (and other internal PIC circuits) to prevent digital
switching noise! Perhaps the xtal's Q is so high that it won't even begin
to decay in only 50uS? (This is my guess, since 50uS is only a few 100
cycles at 4MHz,BUT if this is the case,are we loosing some of the benefit
of sleep mode?)

I know that 2 LSB error isn't that bad,but in my case,it is really annoying
because the actual ADC count gets multiplied by 7 before being displayed on
an LCD display. SO, if I calibrate it to be correct in one portion of the
scale, I get as much as 14 or sometimes even 21 counts (after being
multiplied by 7). Yeah, I know,an 8 bit ADC on a ucontroller is not a
really precision device and multiplying by 7 is making things worse,but I
think I could live with 1 LSB of error more easily. MORE IMPORTANTLY, I
want to make sure I am not doing something dumb with the sleep mode.

Thanks,

Sean


|
| Sean Breheny
| Amateur Radio Callsign: KA3YXM
| Electrical Engineering Student
\--------------=----------------
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