1. Basic Information

Customer's Described Information	The customer reported that during product testing, the board's time stops and then resets, requiring reconfiguration of the time after reset.
Supplementary Information	2 PCS use the 3215/32.768KHz/12.5pF crystal before the board revision, and 3 PCS use the 308C/32.768KHz/6pF crystal after the board revision. It is necessary to verify all parameters of the crystal circuit and issue a report. PCB Board Numbers and Crystal Specifications Used: 1.PCBA/PN: 25010083, 25035377; Crystal Parameters: 3215/32.768KHz/12.5pF 2.PCBA/PN: 25036659, 25035392, 25035249; Crystal: 308C/32.768KHz/6pF Power Supply Mode: USB Power Supply

2. Handling Steps

2.1 Verify the Defective Phenomenon:

Test the PCB returned by the customer. The crystal specification used on the board is: 3215-32.768KHz-12.5pF

X1 oscillation circuit operating frequency FL = -5.4 ppm; Result: OK

X1 series resistance margin |-R| = 90 KΩ (Reference value: |-R| > 3 RR); Result: NG

Test results of the crystal oscillation circuit with 3215/32.768KHz/12.5pF under original board parameters as follows:

X1 oscillation circuit operating frequency FL = -23.10 ppm; Result: OK

X1 series resistance margin |-R| = 90 KΩ (Reference value: |-R| > 3 RR); Result: NG

Tests show that when the 3215-32.768KHz-12.5pF crystal is used with the customer's circuit board, the negative impedance margin is small (|-R| < 3 RR), presenting a risk of oscillation stop.

• Root Cause:

The root cause of the defect is determined to be that the 3215-32.768KHz-12.5pF crystal has a relatively large internal resistance (ESR < 70kΩ). Insufficient negative impedance margin leads to the crystal stopping oscillation during operation on the customer's circuit board, which in turn causes the time reset phenomenon when the device resumes working. The magnitude of negative impedance reflects the load capacity of the IC, not an issue with the crystal itself. Therefore, this is identified as a component selection issue on the customer's side.

• Handling Actions:

Replace the crystal on the defective boards reported by the customer with the specification: 308C-32.768KHz-12.5pF, and re-test.

4.6.1 Test results of the crystal oscillation circuit with modified crystal specification: 308C/32.768KHz/12.5pF as follows:

X1 oscillation circuit operating frequency FL = -11.1 ppm; Result: OK

X1 series resistance margin |-R| > 120 KΩ (Reference value: |-R| > 3 RR); Result: OK

Since the 308C-32.768KHz-12.5pF crystal has a smaller internal resistance than the 3215-32.768KHz-12.5pF one, the test results show that all parameters are within the reasonable range (|-R| > 3 RR). Therefore, it is recommended that the customer replace the crystal with the specification: 308C-32.768KHz-12.5pF.

3. Final Disposal Measures

• It is recommended that the customer change the crystal specification to SJK 308C/32.768KHz/12.5PF. Using this specification ensures the oscillation margin is within a reasonable range, which reduces the risk of oscillation stop caused by insufficient negative resistance margin, and the frequency deviation is small. 

• No further defective feedback has been received from the customer or the sales representative.