手机逻辑电路常见故障（Common failure of mobile logic circuit）.doc

手机逻辑电路常见故障（Common failure of mobile logic circuit） 5. Common faults in logic circuits: 5.1 cannot download program analysis PCBA adopts online DOWNLOAD, and the logic circuit cannot work often causes SN/IMEI and DL failure. Here are three conditions for the logical circuit to work properly: 1. Normal operating voltage: VBAT, RTCVDD, VRF, VDDCORE; The clock is normal: 32.768 KHZ and 26MHZ; Data bus, address bus, read/write, reset, and select signal are normal. When downloading the program, the battery interface is added 3.8 V 1.5 A power source, the CPU detected the NBOOT signal as low level, and the phone entered the NBOOT mode (download mode). The current normal working current is about 90mA. If the current is low, the circuit is not working, and the power supply voltage can be checked. There is no reference clock 26MHZ, RTC clock 32.768 KHZ, its amplitude is normal. If the current is large, it indicates that some of the power supply voltage has short circuit and can be used in the case of uncharged Whether the resistance of the voltage to the ground is abnormal. Current is 0.5 A, common for VSIM, VRF2.85 V to ground short circuit; The current is 1.5a, which causes the power protection, and it is common for VBAT to short circuit, and check the circuit of components related to VBAT. Current such as normal, mobile phone in normal mode, FLASH program, had been written NBOOT test point can detect the pulse signal, the signal can be measured, speaking, reading and writing is normal, after the completion of the DL, cellphone, can measure the pulse signal to the data. 5.2, unable to enter the download status analysis Voltage, current, clock signal is normal, open DOWNLOAD software, can not enter the DL state, can check whether the serial port communication is normal, J5, U100 is false welding, even welding Was the multimeter diode profile measurement (here) can be measured on the J5 TX0, RX0 turn-on voltage of 0.7 V (red pens and grounding, black pens and the test point.