Use of 24 MHz requires a minimum voltage of 4.75V per the datasheet specifications. In fact this applies to any CPU speed above 12 MHz, so using an external clock between 12 - 24 MHz must also adhere to this requirement. Many enCoRe II applications use USB Vbus 5V as the power source for the device. Per the USB specification, voltage can be less than 4.75V on Vbus. Even for externally powered 5V applications, developers should consider that on power up and power down voltage will be less than 4.75V for some time. Firmware must be implemented properly to prevent undesired behavior.
Use of 24 MHz requires the use of the high POR trip point of approximately 4.55 - 4.65V (Register LVDCR 0x1E3, PORLEV[1:0] = 10b). This setting is sufficient to protect the device from problems due to operating at low voltage with CPU speeds above 12 MHz. This must be set prior to setting the CPU speed to greater than 12 MHz. For devices with slow power ramps, changing the POR threshold to the high level may result in one or more resets of the device as power ramps through the chip default POR set point of approximately 2.6V up through the high POR set point.
If multiple resets are undesirable for slow power ramps, then firmware should follow this process:
- Set the Low Voltage Detection circuit (Register LVDCR 0x1E3, VM[2:0]) for one of the set points above the POR (VM[2:0] = 110b ~4.73V or 111b ~4.82V).
- Monitor the LVD until voltage is above the trip point (Register VLTCMP 0x1E4, bit 1 is clear)
- Debounce the indication to ensure that voltage is above the set point for possible noisy supplies
- Set the POR to the high set point
- Shift CPU speed to 24 MHz
If voltage may dip below 4.75V and the application can tolerate running at a CPU speed of 12 MHz, then application firmware may also implement the following to minimize the chance of a reset event due to a voltage transient
- Set the LVD for one of the highest two setting as shown above
- Enable the LVD interrupt
- In the LVD ISR, reduce CPU speed to 12 MHz and shift the POR to a lower threshold
- Firmware can monitor for VLTCMP to clear within the normal application main loop
- Debounce the indication to ensure voltage is above the set point
- Shift the POR to the high set point
- Shift the CPU to 24 MHz