doc: document project

README.md

@@ -1,9 +1,48 @@
-this is a template firmware for [ESP32-S2](https://www.espressif.com/en/products/socs/esp32-s2)-based micro-controller projects.
+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
 =====
 
-the board used is a [WEMOS](https://www.wemos.cc/en/latest/index.html) [S2 mini](https://www.wemos.cc/en/latest/s2/s2_mini.html).
+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
 =================