Here’s a nice little feature-packed USB power meter, the UM25C. This unit has USB-C along with the usual USB type A connectors, along with a bluetooth radio for remote monitoring of stats via a Windows or Android app. Construction is nice, it’s a stack of two PCBs, and polycarbonate cover plates, secured together with brass posts & screws.
The back cover has the legend for all the side connectors, along with the logo.
Down the sides are the user interface buttons, and here the Micro-B input connector. The 4-pin header is visible here that takes serial data down to the bluetooth section.
The other side has the remaining pair of buttons, and the USB-C I/O. I don’t yet own anything USB-C based, but this is good future proofing.
Removing the top plastic cover plate reveals the small 1″ TFT LCD module. This will be hot-bar soldered underneath the screen. There’s an unused footprint next to the USB input connector, judging by the pin layout it’s probably for a I²C EEPROM.
The underside of the top PCB has all the main components. The brains of the operation is a ST STM8S005C6T6 microcontroller. It’s at the basic end of the STM range, with a 16MHz clock, 32K flash, EEPROM, 10-bit ADC, SPI, UART & I²C. The main 0.010Ω current shunt is placed at the top left of the board in the negative rail. A couple of SOT-23 components in the centre of the board, I haven’t been able to identify properly, but I think they may be MOSFETs. The large electrolytic filter capacitor has a slot routed into the PCB to allow it to be laid flat. Providing the main power rail is a SOT-89 M5333B 3.3v LDO regulator.
The bottom board contains the bluetooth radio module, this is a BK3231 Bluetooth HID SoC. The only profile advertised by this unit is a serial port. There’s a local 3.3v LDO regulator & support components, along with an indicator LED.
Since this phone has been in my drawer for some time, I figured it was time for a teardown. (It’s never going to see any more use).
The back cover on these phones is easily removed, as it’s just clipped on.
Once the back cover is removed, the Li-Polymer cell is exposed, along with the logic board. Pretty much all of the PCB is under RF shields.
Under the small RF can on the back of the board is the battery management circuitry & the charger. There’s an extra connection to the cell for temperature monitoring. Just under that circuitry is the eMMC flash storage.
Just to the left of the battery circuit is the NFC transceiver IC, from NXP.
The cell is connected to the main board with a FFC, with a very small SMT connector, although not as small as the more modern Xperia series phones.
The other side of the mainboard holds the large RF transceiver section, with a Qualcomm RTR8600 multiband transceiver IC. In the bottom corner is a Skyworks SKY77351-32 Quad-band power amplifier IC, along with 3 other power amplifier ICs, also from Skyworks.
The top corner of the board holds the various sensors, including an Invensense MPU-3050 3-axis gyro. To the right of that is the Audio Codec, a WCD9310 from Qualcomm.
Everything is controlled from the last section on the board, with the main CPU & RAM in a PoP (Package-On-Package) configuration. Under the main CPU is the main power management IC, also from Qualcomm. No datasheet for this one unfortunately, but it gives it’s purpose away by being surrounded by large inductors & capacitors.