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Revision 1.07

4.6

PS/2 INTERFACE

The PS/2 protocol is an industry-standard, PC-AT-compatible interface for keyboards. It uses a two-wire bidirectional TTL

interface for data transmission. Several vendors also supply PS/2 mouse products and other pointing devices that employ

the same type of interface.

The PC87591x provides four PS/2 data transfer channels. Each channel has two quasi-bidirectional signals that serve as

direct interfaces to an external keyboard, mouse or any other PS/2-compatible pointing device. Since the four channels are

identical, the connector ports are interchangeable.

4.6.1

Features

●

Four PS/2 channels

●

Enable/Disable for each of the four channels

●

Automatic hardware shift mechanism

●

Hardware support for PS/2 auxiliary device protocol

●

Processor interrupts at the beginning and end of data transfer

●

Optional software-based PS/2 implementation

4.6.2

General Description

In the previous generation of keyboard controllers, firmware executed the PS/2 device interface by toggling the interface

signals. The PC87591x (and its predecessor, the PC87570) supports this bit toggling mode via either polling or interrupt-

driven clock edge detection.

PS/2 devices’ firmware is significantly simplified through the use of a hardware accelerator mechanism. The accelerator in-

cludes an 8-bit shift register, a state-machine and control logic that handle both the incoming and outgoing data. It reduces

code overhead, performance requirements and reduces the overall interrupt latency from the core firmware. The hardware

is designed to meet the PS/2 device interface as defined in Keyboard and Auxiliary Device Controller (Types 1 and 2),

August 1988.

Section Naming conventions

●

In this section, the term “channel” describes the interface to one of the PS/2 devices and its two associated signals

(clock and data).

●

The term “shift mechanism” refers to the hardware accelerator.

●

The term “PS/2 interface” refers to the entire mechanism.

Interface Signals

The PS/2 interface includes eight external signals (PSCLK4-1 and PSDAT4-1) and six registers.

Module Block Diagram

A schematic description of the PS/2 interface appears in Figure 38. The interface to the three channels is symmetric and

only channel 1 is detailed in the figure.

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