teleterra: comment the audio output implementation a bit better

Signed-off-by: Keith Packard <keithp@keithp.com>

diff --git a/src/ao_audio.c b/src/ao_audio.c
index 70157557eb80bf02717415f719984b26774056e4..0d521973f78cc3b43c4c52939cace53407fb3f60 100644 (file)
--- 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,