1- System is powered by a 3.7V LiPo
2- micro-B USB is used to charge the system. (A very low cost 3 pin connector type)
3- LTC4056 (LTH7) serves as BMS (2.7k prog resistor) with Vpreset of 4.2V. With no battery present, it is down to 3.66V, Shutdown
4- AUX_DET checks for low on T pin to notify insertion.
5- MIC is connected to dedicated port on MCU but FW doesnt seem to use it.
6- System FW forces BT first, even when AUX plugged in.
7- The hole on the corner is probably for FM antenna
8- The Audio AMP is dual mode (Class F/D) but here forced on F
9- Switch inputs are chained to save 1 GPIO for 4 switches
10- A very slow TF Card interface utilizing only 1 data pin.
11- microSD is powered by VDDIO generated internally by MCU (current limited by 4.7k resistor.)
12- The SD detect pin is pulling the SD Card clock. (Best Guess: When no card is present, det is pulled low internally by the connector and hence the clock line has a strong pull down. When card is present, the clock line gets its needed pull-up to VDD)
13- Bluetooth RF connects to a Printed Antenna (2.4GHz) while the FMIP connects to a dipole antenna passing through a pi filter.
14- MIX2018A audio amp is a dual mode (F/D) amplifier serving as class F in this application.
15- The Audio Amp enable/control line is controlled by MCU GPIO
16- MCU serves a BLUE status LED (open drain Output).
17- AUX input jack Left/Right channels are combined.