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THROUGH THE LOOKING GLASS

Taking a Look at the PIC18Cxxx Series
by David Brobst

Start ý PIC18Cxxx Chips ý Memory ý Data Memory ý Advanced Indirect Addressing ý Deep and Accessible Stack ý Interrupts ý Power-On Features ý Clock Speed ý 10-Bit A/D ý Hardware Multiplier ý Timers ý CCP/PWM ý USART ý I²C Master ý Table Read/Write ý Current Status ý Sources and PDF

POWER-ON FEATURES

The PIC18Cxxx family includes a programmable brownout detect and low-voltage detect. The brownout detect levels are set using the configuration word during programming. This is a nifty feature that allows for different brownout levels to be detected depending on the application. This brownout unit uses ~50 ýA, which is a welcome relief from the initial brownout circuits of the midrange devices that gulped down an astounding 425 ýA.

The low-voltage detect is under software control and can be set using a special function register. It can be turned on and off at will and can generate an interrupt, so when a low-voltage condition happens, the event is automatically accounted for. The potential drawback to the low-voltage detect circuit is that it runs off of the internal power rails. There are pluses and minuses to this configuration.

On the minus side, there is no way to directly interface to an external voltage, only the V_DD of the controller. So in a battery system with a step-up regulator, the voltage is read off of the regulated side and not the batteries. One way around this shortfall is to add an A/D channel to measure an external voltage. On the plus side, using the internal rail enables a low-voltage detect without an I/O pin.

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