Gil,

The input stage is probably the easiest thing to hook up wrong. Can you tell us about the telco circuitry and where you tied the dtmf inputs and outputs of the MT8880?

I understand your comment about the INT signal generating as a suggestion that the DTMF tones are possible decoded correctly but that you may be having trouble either in the 8051 interface circuit (which is standardized) or possibly the software interface.

I assume that you have increased the odds of this problem localization by observing that the INT only appears when a proper dtmf tone is applied for it to decode. While the poorly hooked up input signal into the MT8880 could still be responding to the tone, it still might not be capable of correctly identifying the tone.

If the bug description is: Tone Inaccurately Read from the MT8880, it still could be either the input hookup or the interface/code.

Tell us more.

If you have a circuit reference on where you got it, post the URL.

aka j

PS: Just for the record my real name is Zeke Armstrong.