Nuts and Volts - January 2017

overflow by trying to store too many addresses on the stack. You can’t read the stack pointer to tell how many levels have been used, so you have to keep track manually while coding. Reading the fine print of the PIC’s datasheet may be overwhelming, but it will help your coding and debugging!

Putting It All Together

While it is theoretically possible to breadboard this circuit (Photo 3), you’ll really want a circuit board. I’ve built some flexibility into the hardware — particularly with respect to the memory. The pinouts of the 62256 SRAM and 27C256 EPROM are not identical, but only the functions of pins 1 and 27 differ. I added two two-position jumpers on the back of the circuit board (not shown on the schematic for clarity) to swap the functions of these two pins (Write/Vpp and A14) and allow Socket 0 to hold a SRAM or an EPROM. If you have smaller capacity memory chips such as a 6264 8K SRAM or a 27C128 16K EPROM, these can also be used with some caveats. You’d think that the un-needed address pins for these chips would simply be “no-connect,” but sometimes they are used as additional chip select inputs. Check your datasheets carefully and either modify your code to set these inputs as necessary or break a trace or two on the circuit board to set them in hardware.

The schematic and PCB layout support a few other hardware options, including a general-purpose crystal oscillator using a couple of leftover inverters; an external crystal oscillator for the PIC for greater clock accuracy or if you want to run faster than its 8 MHz internal clock limit; access to the clock and interrupt outputs of the RTC chip; and a three-terminal analog temperature sensor. There is also an unused output on the 74HC138 decoder that can be used to access other data bus devices or drive a piezo speaker.

Note that there are no pads for R11 on the circuit board as I just couldn’t find space for it, so you’ll have to solder it across the pins of crystal X3 on the back of the board; the same goes for R12, the pull-up resistor for the open-collector CLKOUT pin of the RTC.

The circuit board holes for the case mounting tabs of J7, the mini-DIN PS/2 keyboard jack are not full size as they are too close to the edge of the board. Use a cutoff wheel or hobby saw to cut notches in the edge of the board to accommodate these tabs.

A potential application for this project is an EPROM programmer or “burner.” One of the Socket 0 jumpers isolates the programming voltage pin on the EPROM The code I’ve written for this project uses a very simple command-line structure to select subroutines that demonstrate the hardware functions described above. Among other things, it will check for the presence of SRAM or EPROM in Socket 0 and adjust the SRAM start address accordingly. I’ve also included a very bare-bones file system to use the EEPROM as a tiny disk drive; its structure is explained in a separate tab of the reference spreadsheet available online.

A 9600 baud serial link with a laptop running a terminal program displays the list of files on the “disk” and supports data import to and export from the SBC. A command reference that explains how each subroutine works is also available online. There are plenty of comments to explain the code and help you roll your own PIC based operating system. So, dust off those vintage chips, read their datasheets, and build a microcomputer to call your own! NV