[sdiy] Autotuning CEM3340/V3340 oscillators

[ASSI]

On Wednesday, September 20, 2017 2:19:59 PM CEST Tom Wiltshire wrote:
> There have been some quirks. One is that I’m running the PIC at 32MHz, and
> consequently have 32MHz or 8MHz available as 16-bit timer clock
> frequencies. Even with prescale settings of /2, /4, or /8, this is too fast
> to measure frequencies below 16Hz. I got around this by keeping track of
> timer overflow, which effectively gives me a 17-bit counter, which is
> *just* long enough for the job. To be honest, no-one will notice tuning
> errors way down there anyway - it’s on the border of sub-audio.

I'd be very surprised if you couldn't chain timers on this chip.  Another 
option (maybe if you need the other timers for different things) at the low 
frequency end is to put an interrupt on the overflow and count them that way.  
Even polling the overflow or MSB might work at those frequencies.

> Another quirk is using a cheap SPI DAC with only 12-bit resolution. There
> are some nice TI 16-bit DACs available in 8-pin DIP, but they cost
> £10/unit, unlike the one I’m using. Obviously I could go SMD, but I’m
> avoiding that.

You'd be better off with integrated DAC anyway.  These are typically also 
twelve bit but much higher update rates, so if you have a timer left, just 
dither the last bit (or two) for some higher resolution.  It's also more 
"analog" if you get the noise characteristics right.  :-)


Regards,
Achim.
-- 
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