Power On Reset (POR)

When a PIC® MCU is first powered up, it will run through some defined hardware functions internally to prepare the MCU for proper operation. This can include the oscillator start-up and power stabilization. The Power On Reset will hold the device in reset while these operations happen. It prevents the device from running any software until a minimum level Vdd voltage threshold is met and the oscillator is stable. POR also handles some internal settings for the hardware to operate properly. Even though the device can be reset by other means, such as the MCLR pin, the internal circuitry will only be reset in this start-up mode with a POR.

The POR can hold the device in reset longer for environments that are known to be noisy by using the Power-up Timer.

The POR threshold has two parameters that are key to proper initiation of a POR. The two parameters include:

  • Vpor is the voltage at which the device will be released from the POR circuitry and allow the device to start operating the hardware startup circuitry.
  • Vporr is the re-arm voltage level that will initiate a new POR. When Vdd drops to this level the POR will be armed and the start-up process will begin exactly the same way a power from completely unpowered would initiate.

These parameters will be specified in the device data sheet electrical characteristics similar to this table below.

porr.png

For older devices, there may only be a Vpor parameter which will represent the voltage level to guarantee a POR will occur. On some older devices the Vpor could be as low as Vss (zero volts). This means that to guarantee a proper POR, the voltage on the device must start from true Vss. A quick drop in the Vdd line at the device close to the Vss level, but not all the way, may not initiate a POR. Another type of reset may occur but not the POR. For voltage sensitive applications such as automotive applications, it is recommend to use the newer devices with the improved POR structure.

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