Skull Driver code.

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Acknowledgements:

The code for reading the .mp3 file is based on a .mp3 player published under the name 'atomic14', with the source at https://github.com/atomic14/esp32-play-mp3-demo. Most of the code in the 'audio' subdirectory is his (as far as I know) with the exception of the 'minimp3.h' file, which is atributed to https://github.com/lieff/minimp3, which was explicitly placed in the public domain by the author.
I don't know the real names of these software engineers, but I am gratefull to them for their work. (I will gladly add their actual names to this notice if I find out who they are, and want me to).

PINOUTS We will use the following controls:

• 2 LEDS, one per light. GPIO 34 (left) and GPIO 35 (right)
• 1 Servo - for the jaw GPIO 13
• 1 Stepper for the nod GPIO1, GPIO3, GPIO21, GPIO19
• 1 Stepper - to rotate the head. GPIO18, GPIO17, GPIO16, GPIO14
• 1 Audio output - GPIO 25 (main channel) and GPIO 26(duplicate or other channel). Audio is driven by a pre-saved sound byte in flash. Flash memory is limited, so we use .mp3 file, 8khz mono. The audio output driver will try and adapt to whatever rate is supplied. Stereo may actually work...

STEPPER The AccelStepper library from the Arduino development library is copied and modified for the ESP environment. The Digital I/O is modified for ESP32, this is configured as a 'driver' - all communication is done via messages to a single callback routine, including timeouts. The 'run' routine is called from the callback only when the next event is expected. Upon completion, the 'time of next step' is calculated, and a event is schedualed in the sequencer.

LEDS Are Driven by the PWM module in the ESP32, using a 'fast' clock.

SERVOS Driven by the PWM module in the ESP32, using a 'slow' clock - 50 CPS (20 msec), varying the duty cycle on comand.

Initial input will be an *.mp3 file.

• .mp3 format audio source, stored in flash memory.

NOTE: There is a bug in the I2S code on master that keeps us from working. As a result we are using IDF version V4.2.2 (branch: release/v4.2).

FUTURE: (Assume audio goes to a separate speaker? Comes from on-board file? Can ESP32 drive a speaker?) The jaw and eyes will respond to amplitude of each block of ?32? bytes