update notes

notes.md

@@ -48,3 +48,50 @@ it turns out you can not put the chip into deep sleep and have it wake up correc
 # EVAL BOARD NOTES
 ## PINOUT
 WCHLinkE SWDIO -> PD1
+
+
+
+# AUDIO NOTES
+## LIMITS
+we have ~9-10kB of space remaining on the chip.
+i can likely squeeze out _maybe_ another kB by dropping every print, but
+that appears to cause some lockup, i think because the SDI peripheral is
+trying to attach, but unable to.
+
+for how things sit currently, we have the following (conservative) limits
+to fit within 9kB:
+
+16ksps, 4b depth -> 64kbps -> 1.125s
+8ksps, 4b depth -> 32kbps -> 2.25s
+6ksps, 4b depth -> 24kbps -> 3.0s
+4ksps, 4b depth -> 16kbps -> 4.5s
+
+in reality, the numbers i have seen are a little smaller than this, likely due
+to compiler optimizations when the dac is unloaded / there's no audio.
+
+a 16ksps file has an _experimental_ max of 0.75s. applying that offset
+everywhere, we get empirical limits of:
+
+16ksps, 4b depth: 0.75s
+8ksps, 4b depth: 1.875s
+6ksps, 4b depth: 2.625s
+4ksps, 4b depth: 4.225s
+
+## FFMPEG
+use the following command to convert an input .wav to a raw mono pcm_u8 with 8ksps:
+```shell
+ffmpeg -i input.wav -f u8 -c:a pcm_u8 -ar 8000 -ac 1 output.raw  
+```
+
+
+# PINOUTS
+LED0:          PC3
+LED1:          PD2
+DAC:           PC4
+COIN SWITCH:   PC2
+BUTTON SWITCH: PD6
+ADC:           PD4
+GPIO:          any remaining pin
+
+## FORBIDDEN:
+PD1