1.4 Residual Power in Decoupling Capacitors

Symptoms:

• Unusual behavior after power cycling
• Failure to restart after applying power (appears as failure to start up)

Troubleshooting:

• Verify that MCLR and RESET are asserted (voltage falls below the threshold)

If sleep modes are used, the microcontroller can draw so little current (in the range of 100s of nA) that the decoupling capacitors are not discharged below the device's Power-on-Reset (POR) threshold, even when removing the power source from the system. The figure below shows a simple example where the Reset time is insufficient to reach the POR thresholds.

Since the POR level never is reached, the device is still running in sleep mode and did not reset. While this can be a problem, this can also be used beneficially. For instance, this may be used to have the microcontroller retain volatile data while changing batteries. However, extra design work is required to utilize this technique reliably.