PC87309VLJ datasheet(162/192 Pages) NSC

162 / 192 page

Mouse and Keyboard Controller (KBC) (Logical Devices 5 and 6)

162

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8.4.2

General Purpose I/O Signals

The P12, P20 and P21 general purpose I/O signals inter-

face to two I/O ports (port1 and port2). P12 is mapped to

port 1 and P20 and P21 are mapped to port 2.

P12 does not exist in the default configuration of Two-UART

mode see “Wake Up Options” on page 19. In this mode,

P12 input is connected implicitly to the port output. It may be

optionally select on either IRRX or MTR1, in which case it is

open drain and not quasi-bidirectional as described below.

P12 is driven by quasi-bidirectional drivers. (See FIGURE

FIGURE 8-6 "Quasi-Bidirectional Driver".) These signals

are called quasi-bidirectional because the output buffer

cannot be turned off (even when the I/O signal is used for

input).

During output, a 1 written to output is strongly pulled up for

the duration of a (short) write pulse, and thereafter main-

tained by a high impedance “weak” active pull-up (imple-

mented by a degenerated transistor employed as a

switchable pull-up resistor). A series resistor to those port

lines used for input is recommended to limit the surge cur-

rent during the strong pull-up. See FIGURE FIGURE 8-7

"Current Limiting Resistor".

If a 1 is asserted, an externally applied signal may pull down

the output. Therefore, input from this quasi-bidirectional cir-

cuit can be correctly read if preceded by a 1 written to output.

P20 and P21 are driven by open-drain drivers.

When the KBC is reset, all port data bits are initialized to 1.

FIGURE 8-6. Quasi-Bidirectional Driver

FIGURE 8-7. Current Limiting Resistor

Port

Write

Internal Bus

PORT

F/F

ORL, ANL

+VCC

PAD

PC87309

Port Pin

R: current limiting resistor

A small-value series current limiting

resistor is recommended when

port pins are used for input.

100

500

Ω

–

100

500

Ω

–

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