4-70

Functional Description of

CDP1871A Terminals

D1 - D11 (Outputs):

Drive lines for the 11 x 8 keyboard switch matrix. These

outputs are connected through the external switch matrix to

the sense lines (S1 - S8).

S1 - S8 (Inputs):

Sense lines for the 11 x 8 keyboard maxtrix. These inputs

have internal pull-down resistors and are driven high by

appropriate drive line when a keyboard switch is closed.

CS1, CS2, CS3, CS4 (Inputs):

Chip select inputs, which are used to enable the three-state

data bus outputs (BUS 0 - BUS 7) and to enable the reset-

ting of the status flag (DA), which occurs on the low-to-high

transition of TPB. These four inputs are normally connected

to the N-lines (N0-N2) and MRD output of the CDP1800-

series microprocessor. (Table 2)

BUS 0 - BUS 7 (Outputs):

Three-state data bus outputs which provide the ASCll and

HEX codes of the detected keys. The outputs are normally

connected to the BUS 0 - BUS 7 terminals of the CDP1800-

series microprocessor.

DA (Output):

The data available output flag which is set low when a valid

key closure is detected. It is reset high by the low-to-high

transition of TPB when data is read from the CDP1871A.

This output is normally connected to a flag input (EF1 - EF4)

of the CDP1800-series microprocessor.

TPB (Input):

The input clock used to drive the scan generator and reset

the status flag (DA). This input is normally connected to the

TPB output of the CDP1800-series microprocessor.

RPT (Output):

The repeat output flag which is used to indicate that a key is

still closed after data has been read from the CDP1871A

(DA = high). It remains low as long as the key is closed and

is used for an autorepeat function, under CPU control. This

output is normally connected to a flag input (EF1 - EF4) of

the CDP1800-series microprocessor.

DEBOUNCE (Input):

This input is connected to the junction of an external resistor

delay (t

≅ RC) after the release of a key. If a debounce is not

desired, the external pull-up resistor is still required.

ALPHA, SHIFT, CONTROL (Inputs):

A high on the SHIFT or CONTROL inputs will be internally

latched (after the debounce time) and the drive and sense

line decoding will be modified as shown in Table 3. They are

normally connected to the keyboard, but produce no code by

themselves. The SHIFT and CONTROL inputs have internal

pull-down resistors to simplify use with momentary contact

switches. The ALPHA input is not latched and is designed for

a standard SPDT switch to provide an alpha-lock function.

When ALPHA = 1 the drive and sense line decoding will be

modified as shown in Table 3.

negative supply voltage terminal and is normal connected to

ground.

All

outputs

swing

from

TABLE 1. SWITCH INPUT FUNCTIONS

CONTROL

SHIFT

ALPHA

KEY FUNCTION

0

0

0

Normal

1

X

X

Control

0

1

X

Shift

0

0

1

Alpha

NOTE:

X = Don’t Care

CDP1871A, CDP1871AC