Tones are joined by writing the location and length registers , starting the audio output, and rewriting the registers in preparation for the next audio waveform that you wish to connect to the first one. This is made easy by the timing of the audio interrupts and the existence of back-up registers. The location and length registers are read by the DMA channel before audio output begins. The DMA channel then stores the values in back-up registers. Once the original registers have been read by the DMA channel, you can change their values without disturbing the operation you started with the original register contents. Thus, you can write the contents of these registers, start an audio output, and then rewrite the registers in preparation for the next waveform you want to connect to this one. Interrupts occur immediately after the audio DMA channel has read the location and length registers and stored their values in the back-up registers. Once the interrupt has occurred, you can rewrite the registers with the location and length for the next waveform segment. This combination of back-up registers and interrupt timing lets you keep one step ahead of the audio DMA channel, allowing your sound output to be continuous and smooth. If you do not rewrite the registers, the current waveform will be repeated. Each time the length counter reaches zero, both the location and length registers are reloaded with the same values to continue the audio output. Audio DMA Example
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