November 2000 , Issue 124

A PIC17C44-Based Computer

TESTING THE BOARDS

When testing, connect power to J12 on the daughterboard and test for 5 V on J13. Check for shorts across pins 1 and 2 of J1 on both boards. Also, connect J13 to J1 on both boards. Connect J2 on both boards with a 34-wire ribbon cable with header connectors.

With no chips in any of the sockets, test for proper voltages as follows. Check for 5 V on the 5-V pins and on other pins connected to 5 V (see Figures 1–3). Then, connect the positive lead of a voltmeter to 5 V and use the negative lead to check for a ground on the GND pins (see tables) and other pins connected to ground (see figures).

Program a PIC from tryports.hex. Install U1 and Y1 in their sockets on the motherboard, but no other chips. Apply power and check for about 2.5 V on in 20. Ports B, C, D, and E should be counting. In addition, check for about 2.5 V on all pins of all sockets where these signals should appear. An oscilloscope should show a pulse train on all these pins, doubling in period with each higher bit of each port.

Next, install the other chips in their sockets on the motherboard. Program a PIC from memory.hex. When power is applied, all bytes of all three memory chips will be written and verified. Port B’s bits all should output a square wave at decreasing frequency as probed from low to high order (use a voltmeter).

RA<2> should pulse low at a frequency of 14 Hz with a 20-MHz crystal in Y1. If a failure occurs, the program will freeze and RB<7:0> will output the bits read. And RA<2> will be high if program memory failed or low if data memory failed. Also note that RA<3> will be pulled to ground if the low-order byte failed. If that doesn’t occur, RA<3> will be hi-Z.

Program a PIC from devices.hex. The CS lines (pins 6, 8, 10, 12, and 14 of J2) will go low sequentially for one program cycle (Fosc/4).

You must assemble the program for the PIC12C508A from aux17C44.asm, but first change the calibration constant to what you just determined. Program PIC12C508A from aux17C44 .hex and install it and the other chips in their sockets on the daughterboard. Then, run additional tests using the supplied .asm and.ext source code files following the next instructions.