P16

Z80 Build From Scratch

PAGE 16: PPI Overview & LEDs

82c55A PPI Overview:

The 82C55A Programmable Peripheral Interface (PPI) is a highly versatile and popular integrated circuit, often used to bridge the gap between a microprocessor (CPU) and external peripheral devices like keyboards, displays, or A/D and D/A converters.

It features 24 programmable I/O lines, organized into three 8-bit ports: Port A (PA), Port B (PB), and Port C (PC), which can be configured by the system software. Port C can be split into two 4-bit ports and can be combined with either Port A or Port B to form 12-bit ports if needed.

The 82C55A offers flexibility through three primary operating modes:

- Mode 0 for simple input/output operations without handshaking

- Mode 1 for strobed I/O with handshaking signals to synchronize data transfer between devices of different speeds

- Mode 2 which configures Port A for bidirectional bus communication.

For the LED bar graph display we'll use the simple Mode 0 operation.

We discussed using the 74138 back on Page 11 to select our I/O devices. Address $00 is used by the UART, address $08, etc., will be used by the PPI.

I/O Addressing Logic

(Click to enlarge)

To address each PPI port individually, addresses with a base of $08 will be assigned. As an example Port A will be $08, Port B will be $08 + $01 or $09, Port C will be $08 + $02 or $0A, and the CWRCfg configuration register will be $08 + $03 or $0B.

82c55A Expansion Board

Below are the schematics and board design for expansion of our Z80 UART system to include a PCB containing the PPI and two circuits.

- LED bar graph

- 4 line by 20 LCD panel which we'll look at later

We're going to test the PPI by outputting data on Port B to a LED bar graph.

PPI Expansion

(Click to enlarge)