diff options
| author | Keith Packard <keithp@keithp.com> | 2010-10-31 00:49:34 -0700 |
|---|---|---|
| committer | Keith Packard <keithp@keithp.com> | 2010-10-31 00:49:34 -0700 |
| commit | 6b17b75ed5dc4e4fcc7278c715479bb921b644d5 (patch) | |
| tree | a548a6e1555f799db4353b602d7c0ca69915b054 | |
| parent | 3493b456399bcc0ea43d28fd4105928bd8793cff (diff) | |
teleterra: comment the audio output implementation a bit better
Signed-off-by: Keith Packard <keithp@keithp.com>
| -rw-r--r-- | src/ao_audio.c | 13 |
1 files changed, 10 insertions, 3 deletions
diff --git a/src/ao_audio.c b/src/ao_audio.c index 70157557..0d521973 100644 --- a/src/ao_audio.c +++ b/src/ao_audio.c @@ -586,7 +586,9 @@ ao_audio_send(__xdata uint8_t *samples, uint16_t nsamples) __reentrant /* * Turn on timer 4. Free running from 0 to 0xff, with P2_0 - * on interval controlled by data written to T4CC0 + * 'on' interval controlled by the value in T4CC0. Run this + * timer as fast as possible, resulting in a frequency of + * 24e6/256 or 93750Hz. */ T4CCTL0 = TxCCTLy_CMP_SET_UP_CLEAR_DOWN | TxCCTLy_CMP_MODE_ENABLE; T4CC0 = 0x7f; @@ -595,7 +597,11 @@ ao_audio_send(__xdata uint8_t *samples, uint16_t nsamples) __reentrant P2SEL = (P2SEL & ~P2SEL_SELP2_0_MASK) | P2SEL_SELP2_0_PERIPHERAL; /* - * Turn on timer 3 to clock data to the PWM output + * Turn on timer 3 to clock data to the PWM output. This + * runs at 24e6/(17 * 128) or 11029Hz, almost exactly the + * desired 11025Hz. Each cycle of the counter, a DMA transfer + * reads a sample and writes to the T4CC0 register, which + * controls the pulse width of the final audio signal */ T3CCTL0 = TxCCTLy_CMP_SET_UP_CLEAR_DOWN|TxCCTLy_CMP_MODE_ENABLE; T3CC0 = 17; @@ -603,10 +609,11 @@ ao_audio_send(__xdata uint8_t *samples, uint16_t nsamples) __reentrant while (nsamples) { uint16_t this_samples; + /* Sigh. The DMA engine has only 13 bits of length */ if ((this_samples = nsamples) > 0x1fff) this_samples = 0x1fff; /* - * Set up DMA from the samples to T4CC0, triggered by T3 CH0 + * Set up DMA to write samples to T4CC0, triggered by T3 CH0 */ ao_dma_set_transfer(ao_audio_dma, samples, |