A general PIC box


For my own experiments I've built a box containing a PIC16C84 and a LCD-display. It's reprogrammable "in system" via external I/O connector, 9-pin female D-SUB.

Schematics

[Schematics] Bad quality. Get it in PostScriptTM instead.

In-system programming

Just by putting the IC-socket in a "normal" PIC-burner and plug the connector.I am able to reprogram the unit by an eyeblink.
The in-system capabilities is primary implemented by two protective diodes, namely D1 and D2. D1 is as indicated in Microchips paper. D2 is a protective diode so one don't break the 78M05 voltage regulator. The 78XX-series is very sensitive to reverse voltage.
Then are of course the correct pins available in in the external connector, namely MCLR, RB6, RB7 and of course the power.

External I/O

The few pins left after the display has been connected are brought out to a 9-pin D-SUB. The reason for choosing RB0/INT as an external pin is (of course) the interrupt capabilities.
No protective circuits are used since this is an experimental circuit for my own purpose. Maybe a reason to have protective circuits :-). OK, I blame it on "it's no commercial thingie".
To be able to controll the backlighter of the LCD I inserted a resistor and transistor. The little strange connection and use of a PNP is just because RA4 is an open-collector output. OK I blew away the RTCC capabilities, but sometimes you have to sacrifice something to get a funny function. The backlight capability has showed to be a good "debugging tool".

Power Supply

The power supply circuits has some special bells and whistles. In using D2 as a protective diode I loose about 0.7-1 volt, which really doesn't matter for the PIC I think. To compensate for the reduction of voltage I have connected D3 on the 78M05 reference line, which raises the 5 volt output to 5 volt plus diode voltage loss. Ie I have 5 volt after the protective diode D2.
LCD's often needs negative supply for the contrast, at least bigger ones. I then created a "virtual" ground with a zener diode called U3 in my drawings. The LCD I choose (16x1) didn't seem to need negative power so I haven't really tried it out. On my prototypes (3) I've just shorted the zener diode.

Circuit board

I drew a circuit board from the schematic, but seems to have misplaced the files. I wanted a circuitboard as big as the 16x1 LCD so the computer module could ride piggy back on the display. In using surface mounted components (except crystal, 78M05 and connectors) I only managed to fill half the available space. Ie it could easily disappear in your palm if you wanted to.
The circuit board is 115x40 mm and components fill 53x40 mm. And it's not compressed in any sense.
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[Copyright © 1996, 1997, 1998 Stefan Petersen, spe@stacken.kth.se ]