this project is about interfacing the GM-1352 sound level meter using an ESP32-S2-based board, so to read the sound level measurements digitally.

GM-1352
=======

the GM-1352 is the successor of the GM-1351, and very similar to it.
the main difference is that the power is not delivered by a 9V battery, but by 3x1.5V AA batteries.

[CuVoodoo #030](https://www.cuvoodoo.info/cuvoodoo-030-audio-is-killing-the-music/) already explained how to interface it.
basically just tap on the lines between the MCU (a EFM8BB10F8G-QFN20) and LCD controller (a Chip-on-Board under a blob of epoxy).
there are test points available for that:

- CS: chip select, to start data communication
- DATA: the actual data bits
- WR: the data clock (not periodic)

there are additional test points to read (or assert) button presses:

- 1: HOLD button (press low)
- 2: MIN/MAX button (press low)
- 3: POWER button (press low)

there is another test point marked S, but I don't know what this is for.

also note that the LCD has segments for Bluetooth and USB.
there might be variants of the GM-1352 with these options.

board
=====

this repository is the firmware for [ESP32-S2](https://www.espressif.com/en/products/socs/esp32-s2)-based [WEMOS](https://www.wemos.cc/en/latest/index.html) [S2 mini](https://www.wemos.cc/en/latest/s2/s2_mini.html) board.

connect the GM-1352 to the S2 mini as follows:

- B+ to 5V: to power or get power from the device
- B- to GND: ground reference
- POWER to 16: to wake up the S2 when the device is powered (the S2 will shut down shortly after the GM-1352)
- DATA to 18: this will be the SPI MOSI line to read the measurments
- CS to 33: this will be the SPI CS line to identify transactions
- WR to 35: this will be the SPI SCK line to clock the data bits (not periodic)

I soldered wires to the pads, and broke them out on a 8-pin female header, located above the battery compartment.
this also to easily connect on row of the S2 mini to it on the back of the GM-1352.

now you can power the sound level meter through the S2 mini USB port, and read the measurements over its serial CDC ACM interface.

compile and flash
=================

the firmware uses [ESP-IDF](https://github.com/espressif/esp-idf) [v5.0](https://docs.espressif.com/projects/esp-idf/en/v5.0/esp32s2/index.html).
See [getting started](https://docs.espressif.com/projects/esp-idf/en/v5.0/esp32s2/get-started/linux-macos-setup.html) to install the IDF.
once installed, load the IDF tools:

~~~
get_idf
~~~

to build the firmware:

~~~
idf.py build
~~~

note that the first time it will download additional components (such as [tinyUSB](https://git.cuvoodoo.info/kingkevin/espressif_tinyusb) and [esp_tinyusb](https://git.cuvoodoo.info/kingkevin/espressif_idf-extra-components/src/branch/esp_tinyusb_dfu/usb/esp_tinyusb)).

the firmware supports [USB DFU](https://usb.org/document-library/device-firmware-upgrade-11-new-version-31-aug-2004) flashing.
it is meant to be used along the [ESP32-S2 DFU mode firmware](https://git.cuvoodoo.info/kingkevin/esp32-s2_dfu).
Once the DFU mode firmware is installed (on the `factory` partition), you can flash this firmware (on the `ota0` partition) using `dfu-util`:

~~~
dfu-util --device 1d50:617b --alt 1 --download build/main.bin --reset
~~~

if the firmware is stuck, and you can't switch from runtime mode to DFU mode using `dfu-util`, hold pin 14 high while resetting.
this will force booting the DFU mode.

you can also flash using the ESP ROM bootloader (hold GPIO0 low while resetting) and `esptool`:

~~~
esptool.py -p /dev/ttyACM0 --before no_reset --after no_reset --chip esp32s2 write_flash --flash_mode dio --flash_size detect --flash_freq 80m 0x50000 build/main.bin
~~~