November 2000 , Issue 124

A PIC17C44-Based Computer

THE PIC12C508A

A PIC12C508A is shipped with a calibration constant in ROM at 0x1FF, coded as a MOVLW instruction. Before erasing a new PIC12C508A, read the program memory with a PIC programmer and note the hex value of the low-order byte of the code at 0x1FF.

The PIC12C508A must be configured for an internal 4-MHz RC oscillator. Although the frequency is not exactly 4 MHz, it is close if the calibration constant is stored in OSCCAL.

MAKING THE BOARDS

The first step in construction is to make the PC boards. You need a negative for each side of the two boards. You can make your own with Kepro RF-2024 reversing film or can have them made at a photo shop that makes lithographic negatives. You can download the artwork from the Circuit Cellar web site.

Download all mirror.lsr files to a printer (files can’t be displayed or printed by Windows). To make your own copies, use transparency film for figure5.lsr–figure9.lsr. If you’re not making negatives, print the files on paper.

You will need two-sided photosensitized boards, such as Kepro S2-712G. Registration is critical during exposure. Using a wooden board as a base, place a blank PC board on the base and one of the negatives on top of the board, emulsion side down, so that the pattern is within the borders of the board. Using a sharp awl, mark the board at one of the two registration hole pads marked with crosses and remove the negative. Then, with a no. 60 bit, drill the board at the mark.

Remove the debris and again place the negative on the board and use a plastic-headed bulletin board tack to hold it in place. Mark the board at the other registration hole pad and the two mounting hole pads at the opposite edge and drill the registration hole. Place the negative on the board and hold it in place with two tacks. Put a clean sheet of glass on top of the negative to hold it in contact with the board, with the edge against the tacks. Expose the board according to the directions for the material used. Then, remove the glass and negative, flip the board over, and repeat the process with the other negative, making sure the emulsion side is down.

Enlarge the two holes to 5/16" and drill 5/16" holes where previously marked at the other two corners. Place 1/4" 6–32 machine screws in the holes and secure them with machine nuts. This allows the developer and etchant to flow under the board. Develop, etch, and tin the board, then remove the machine screws. Use a no. 60 bit to drill holes through all pads. If you want to mount the auxiliary boards separately, use a hacksaw to cut the boards apart along the guidelines.

Next, solder the feedthrough conductors. Use 24-gauge bare tinned-copper wire. Be careful not to solder a conductor in a pad intended for a component lead. Feedthroughs always connect traces on both sides of the board, but some pads are intended for component leads. Because none of the feedthroughs are under a component, they can be soldered after the components. However, that approach is more difficult. Be sure to make good solder connections, because poor connections can be difficult to trace.

After that, solder the components starting with the smallest and proceeding according to size, again avoiding bridges. Do not solder ICs directly to the board, instead, use sockets. For development purposes, it is advantageous to use a ZIF socket for U1. I recommend the JDR Microdevices 40-6554-10 socket. Remove the rosin with acetone. Do not immerse the board; use a small brush and let the acetone drip off the edge. When you’re finished, check for solder bridges and unsoldered pads.