Hardware : EB004 - LED Board

Symptoms :

The 'knight rider' flashing LED program did not light LED LA4 when it 'scanned' the LED's.

Cause :

From the PIC16F84A datasheet:"The RA4 pin is a Schmitt Trigger input and an open drain output. All other RA port pins have TTL input levels and full CMOS output drivers."

Solution :

What this means in practice is that the LA4 LED on the Matrix PICmicro Development Board will not light if the RA4 pin is output high *unless* the button SA4 is pressed. Pressing this button effectively adds a pull-up resistor to the output of pin RA4, which is required to drive the LED. We recommend that you only use RA4 as an input unless you understand the implications of its open drain output.