iPhone 6 firmware version was gone, how can i get it back?

The iPhone 6 has been dropped before, the rear cover was bended, but it can use normally. We all want to use the perfect thing, so we replace the rear cover. Unfortunately, then we cannot find the firmware version in "about iPhone". Something wrong, we need to find the problem, and fix it.

1. Disassemble the iPhone 6, remove the motherboard. Observing from the side face, we can find that the board bends. We presume that the baseband or the baseband power supply is pseudo soldering. Energizing it and turn it on to test the power supply. VREG_SMPS2_1V25, VREG_SMPS3_0V95 and VREG_SMPS4_2V075 are normal. Only VREG_SMPS1_0V90 has no voltage. The circuitdiagram of these voltages is shown in picture 1.

The test points of VREG_SMPS2_1V25, VREG_SMPS3_0V95 and VREG_SMPS4_2V075 in bitmap are shown in picture 2. They are all measured at the inductors around the baseband.

2. According to experience, I cannot measure the baseband at CUP, which suggests that will cause no voltage on VREG_SMPS1_0V90. As the board bends slightly, we tear down the baseband and test it carefully, but I do not find any dropping pins. I continue to measure the diode data of I2C bus BB_EEPROM_SDA and BB_EEPROM_SCL that act as communication between baseband and baseband chip. The data is 650, which is normal. The circuit diagram of I2C bus is shown in picture 3.

3. Measure whether the communication circuits between CPU and baseband are normal or not. Use multimeter to measure the diode data that baseband flows to CPU signal circuits, the test points are shown in picture 4. We can find that the diode data of the two yellow points are infinite, as show in picture 5. They are the UART bus signals that run to CPU, named BB_CTS_L and BB_UART_RTS_L.

                                               

4

4. Tear down the CPU, and make sure whether it is pseudo soldering or disconnecting. After tearing it down and measuring, we find that the two solder pads are connecting to the yellow points in picture 5. So CPU is pseudo soldering. After reassembling CPU, we can find the firmware version in “about iPhone”. 

 

5

Finally, the problem of iPhone 6 is removed and it back to normal work.