Chip I2C communication exception can not read and write register crack

Clock chip PCF2129 in the mass production process, there are customer feedback PCF2129 has about 0.2% of the chip I2C communication abnormality, can not read and write registers, chip BAT power supply is lower than VCC or VCC and BAT power supply to restore normal. After testing, the 0x02 address register value is abnormally 0x88, and the default value is 0x00, and the chip returns to normal.

The clock chip is used in many designs. The PCF2129 is a built-in crystal high-precision clock chip from NXP. The chip has a compensation register with an accuracy of 3ppm. The clock chip has the characteristics of high precision, low power consumption, and convenient use.

In use PCF2129, a customer feedback occurred in 0.2% of the chips on the battery power, I2C abnormalities such as read and write registers in FIG. 1, a communication failure, the battery is disconnected and then connected after the chip resumes normal operation. If the BAT supply voltage is lower than VCC, the chip can communicate, but the BAT voltage is higher than VCC, and the communication is abnormal. The customer VCC uses 3.3V. The battery is also 3.3V.

Figure 1 PCF2129 has no ACK response

From the analysis of the customer feedback, the chip resumes normal after the chip is re-initialized after the new power-on. After commissioning BAT voltage lower than the voltage VCC, the communication can be normally Figure 2, it can be seen, the chip should be abnormal values deviate from the default value of a register, the register value and the supply voltage and the relevant BAT. In PCF2129, only the address register 0x02 and on the supply voltage as BAT FIG.

Figure 2 PCF2129 IIC communication is normal

Figure 3 PCF2129 0x02 register

The default is 0x00 0x02 register, in case an external power supply BAT continue, after the VCC voltage has been adjusted higher than BAT, the IIC communication is normal, the read register value 0x02 as shown in FIG. 4, is 0x88.

Figure 4 0x02 register outliers

After the PCF2129 is powered off, or directly change the 0x02 register value to 0x00, the PCF2129 I2C is normally normal and the chip works normally.

Figure 5 0x02 register default value

From the test results, it can be concluded that the PCF2129 I2C communication abnormality is caused by the abnormal value of the 0x02 register value. After being modified to the default value, the PCF2129 can work normally.