PIC Failures

PIC Failures

I have been working with PIC microcontrollers almost continuously for 5 years and in that time I have broken one PIC16F type. What that tells us is that it is extremely difficult to destroy a PIC providing that the power supply is current limited to a low current. The 5 volt power supply for the Brunning Software plugboard is limited to about 35 mA which is the optimum compromise.

From time to time customers telephone to ask my advice about faulty PICs and more often than not the problem is that the PIC is being used incorrectly. So here is a list of the common mistakes:-

1. RB3 does not operate correctly when used as an input or an output pin.

The PIC maybe configured with low voltage programming turned on. RB3 is used as the low voltage programming control pin so if low voltage programming is on RB3 cannot be used as a normal input/output pin.

2. RA4 operates correctly as an input but not as an output.

When RA4 is configured as an output it operates as an open drain output. RB4 is the only output which has this characteristic and it is ideal where you need an output which is either grounded or open circuit. If you need RB4 to behave as a normal output you must use a pull up resistor.

3. The PIC programmes and verifies correctly but does not run the programme.

Before a new PIC can be used it must be programmed with the code that you want it to run, then it must have its configuration fuses set. If a PIC programmes correctly with a known good programme but will not run the code it is possible that it has not been configured or that it has been configured incorrectly.