The T-Watch 2020 sound system

The t-watch has an integrated loudspeaker which is be driven by a MAX9835a PCM Input Class A Audio Power Amplifier. This system allows us to play wav files on the t-watch.

The MAX9835a uses I2S communication invented by Philips in the late 1980 ies for the transfer of digital sound samples. The protocol uses 3 signals:

The software

We use the I2S driver developed by M. Teachman. Unfortunately this driver has not been integrated into the MicroPython sources and a few modifications to the ESP32 port must be made to include it:

• first the driver itself (machine_i2s.c) must be copied to ports/esp32

• { MP_ROM_QSTR(MP_QSTR_I2S), MP_ROM_PTR(&machine_i2s_type) },

  must be included into

• STATIC const mp_rom_map_elem_t machine_module_globals_table[] = {

  in modmachine.c

• extern const mp_obj_type_t machine_i2s_type;

  must be included in modmachine.h

• machine_i2s.c \

  must be added to the C sources SRC_C
  in the Makefile in order to compile the module into the MicroPython binary.

import machine.I2S

Enabling the sound system

In the t-watch the MAX9835a is powered through the AXP202 power manager and the power line to the sound chip must be enabled before using it. Here is the initialization procedure I use:

from machine import Pin,I2S
import ttgo
from axp_constants import AXP202_VBUS_VOL_ADC1,AXP202_VBUS_CUR_ADC1,AXP202_BATT_CUR_ADC1,AXP202_BATT_VOL_ADC1

watch = ttgo.Watch()
watch = watch
tft = watch.tft
power = watch.pmu
power.adc1Enable(AXP202_VBUS_VOL_ADC1
                      | AXP202_VBUS_CUR_ADC1
                      | AXP202_BATT_CUR_ADC1
                      | AXP202_BATT_VOL_ADC1, True)
watch.lvgl_begin()
watch.tft.backlight_fade(100)

print("enable power")
watch.enable_audio_power()

Mike Teachman's example programs with the power initialization added in, have been uploaded to https://github.com/uraich/twatch2020_firmware/tree/main/hardware/sound.

Before running these however you should create a directory /wav on the ESP32 file system and copy the sound file

https://github.com/miketeachman/micropython-esp32-i2s-examples/blob/master/wav_files/taunt-16k-16bits-mono-12db.wav to this directory.

-- Uli Raich - 2021-01-25

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