Some good news! Using a square wave oscillator to provide loop functionality does indeed provide more stable operation when ‘loop’ is active whilst recording. The only downside is that the oscillator is bleeding into the audio input of the ISD1820, which results in a high pitched whining sound being recorded. I suppose the other downside is that several inverters of the CD40106 are redundant. Is that really a problem? Well, perhaps there might be a simpler way to create an oscillator.
The granular effect I was hoping for wasn’t achieved. The oscillator was producing a square wave which was connected to the play trigger pin of the ISD1820. This oscillator is running at such a high frequency that once recording ceases it’s only a fraction of a second before the play trigger pin receives a rising edge required to begin playing. Whilst the sample is playing, the loop oscillator continues to send rising edges to the play trigger pin. This doesn’t seen to be an issue as the sample continues to play to the end at which point it’s ready to receive another rising edge and begin playing again, this gives the impression of looped playback. I experimented by connecting a loop oscillator to the play gate pin. The sample will only play when +5 volts is present at this pin. When the loop oscillator waveform drops to 0V the sample stops playing. As the loop oscillator is running at a high frequency all that was heard was a series of clicks. A granular effect should be achievable with control over pulse width. I’m in two minds about implementing this as it will complicate the interface somewhat as to get a basic loop effect as before one would need to set both the frequency and pulse width of the oscillator accordingly.
I went back to ‘Getting Started in Electronics’ to reference a touch switch circuit which utilises a CD4011 IC. I didn’t have a 22m resistor as suggested by the schematic but found a combination of a 22r and 100k resistor sufficed. The touch switch was wired up to the record pin of the ISD1820 and works well. I’m thinking now that perhaps the oscillator and touch switches can be provided by a single AVR chip. Perhaps an ATTiny85 has too few pins for the functionality I have in mind? Some further thought is required.