November 2000 , Issue 124

A PIC17C44-Based Computer

THE MOTHERBOARD

The motherboard features a 16-bit address bus, 16-bit data bus, 8 KB of external program memory (ROM or RAM), and 8 KB of data memory (RAM) (see Figure 1). A 34-pin header provides access to the low-order 8 bits of the address bus and data bus, 10 I/O pins, five chip-select lines for external devices, and the OE and WR lines. A 6-pin header provides for connection to a serial I/O level converter.

The three high-order bits of the address bus are used to select the memory chips or external devices. 2000:3FFF selects the 16-bit program memory; 4000:5FFF selects the 8-bit data memory. 6000:7FFF, 8000:9FFF, A000:BFFF, C000:DFFF, and E000:FFFF activate CS3, CS4, CS5, CS6, and CS7, respectively.

J1 provides power from a 5-V supply. You can connect a reset button to the third and forth terminals. J2 is a 34-pin header that connects the motherboard to a daughterboard. It provides connections to the buses, I/O lines, and control lines as discussed above. J3 is a 6-pin header used to connect to J5 or J7 on the daughterboard for serial I/O level conversion.