The PIC-EL was designed for the 16F84 ...
by Craig Johnson, AA0ZZ 5/24/2004
Two minor hardware changes are needed in the PIC-EL board (Rev A) to make it work with a 16F628 PIC.
1. Some PIC-EL boards will not program a P16F628 with the LCD in place. The 16F628 (unlike the 16F84) is very sensitive to current flow into PIC Pin 10 (RB4) when programming. The 16F628 uses this pin for "low voltage programming", a function which is not implemented in the PIC-EL because it doesn't apply to a 16F84. This pin (RB4) is used in the PIC-EL as the "Enable" signal for the LCD. However, when the 16F628 goes into programming mode, RB4 is configured as an Input, and the LCD can source a bit of current into the pin. This can be enough to make the 16F628 fail to program.
Since the LCD is not used in all projects, you could decide to remove the LCD and this change would not be necessary. However, you will probably want to use the 16F628 in future projects that use the LCD so you do not want to remove it. This means you need to make this simple modification:
FIX: Add a 10K resistor from PIC Pin 10 to ground. This can be done easily on the bottom of the PIC-EL board by soldering one end of a 10k resistor to PIC pin 10 and the other end to the ground side of capacitor C10. No shrink wrap necessary. since it is very close. As an alternative, you could solder the resistor between the LCD socket pins 1 and 6.
2. Another modification to the PIC-EL board is needed to make it work reliably with a 16F628. With the current configuration, PIC Pin 1 is used for two purposes. It can be an output (feeding the speaker via transistor Q5) or it could be used as an input, making Pushbutton 3 usable. A pull-up resistor, R34, is used to make the pin HIGH when configured as an input. Unfortunately, because of the speaker circuitry connected to this pin as well, the 10k R34 pull-up resistor only pulls the pin up to 2.75v or so. This is high enough to be recognized as a "HIGH" by a 16F84, but unfortunately, a 16F628 will still see this as a "LOW", making it look like the pushbutton is always being pressed.
FIX: We need to make the pull-up resistor, R34, smaller. There are two easy options here.
a) Remove R34 and replace it with a 5.6k resistor, or
b) On the bottom of the PIC-EL board, solder another 10K resistor across the R34 connectors, making the resistance equivalent to 5k.
With these hardware modifications, the PIC-EL will program the 16F628 and the PIC-EL is able to use the 16F628 for your projects.
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Page last updated: October 1, 2005