567s are notoriously unstable.

Joe M.

Dwayne Reid wrote:
> At 12:31 PM 3/26/2010, M. Adam Davis wrote:
>> Have you looked at implementing it in a low power processor, such as a 16F722?
>>
>> They consume under 500uA running at 1MHz.  Not sure what the ADC
>> consumes running at 4kHz (DTMF highest tone is 1,633Hz), but you may
>> be able to meet your needs by running the goertzel algorithm discussed
>> on the list a bit ago:
>>
>> http://www.google.com/search?q=goertzel+tone+detection
>>
>> Or the more efficient DTMF detections routines discussed in this TI
>> msp430 app note:
>>
>> http://www.google.com/search?q=slaae16
>>
>> You may get even lower power with a PIC18F13K22 or similar since you
>> can clock it more slowly and use the multiplier for whichever
>> algorithm you choose.
> 
> Many thanks for the suggestion.  Unfortunately, I don't have the time 
> to learn how to write this code right now - I have to deliver 22 
> finished, working boxes in 3 weeks.  Yeah - its a small project but 
> its for a good customer.
> 
> Does anyone have code snippets available?  Links to existing 
> code?  I'd like to learn how to do this.
> 
> Luckily for me, my customer just relaxed his requirements.  Now I 
> need to detect only a single DTMF tone pair.  First thought was a 
> couple of NE567 tone decoder chips but the supply current for even 
> just one of those is higher than the mt8870.  But: a pair of cd4046 
> CMOS PLLs might just do the trick.  Add a cd4001 quad NOR gate for 
> lock detection - and I'm done.  I'll start playing with this on Monday.
> 
> That said - I *still* want to learn how to do this on a micro-controller.
> 
> Many thanks!
> 
> dwayne
> 
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