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Function Pin Definition

CLOCK (Input):

Positive input pulse that latches READ and CS on its trailing

edge.

CS - CHIP SELECT (Input)

A high-level voltage at this input selects the CDP1851 PlO.

RA0 - REGISTER ADDRESS 0 (Input):

This input and RA1 are used to select either the ports or the

control/status registers.

RA1 - REGISTER ADDRESS 1(Input):

See RAO

BUS 0 - BUS 7:

Bidirectional CPU data bus.

CLEAR (Input)

A low-level voltage at this input resets both ports to the input

mode, and also resets the status register, A RDY, B RDY,

and interrupt enable (disabling interrupts).

A INT - A INTERRUPT (Output):

A low-level voltage at this output indicates the presence of

one of the interrupt conditions listed in Table 3. This output is

an open-drain NMOS device (to allow wired O Ring) and

must be tied to a pullup resistor, normally 10k

Ω.

B INT - B INTERRUPT (Output):

A low-level voltage at this output indicates the presence of

one of the interrupt conditions listed in Table 3. This output is

also an open-drain NMOS device and must be tied to a

pullup resistor.

B RDY - B READY (Output):

This output is a handshaking or data bit I/O line in the bit-

programmable mode.

B STROBE (Input):

An input handshaking line for port B in the input and output

modes, and for port A when it is in the bidirectional mode. It

can be used as a data bit I/O line in the bit-programmable

mode except when port A is not programmed as bidirectional.

B0 - B7:

Data input or output lines for port B.

Ground

A0 - A7:

Data input or output lines for port A.

A STROBE (Input):

An input handshaking line for port A in the input, output, and

bidirectional modes. It can also be used as a data bit I/O line

when port A is in the bit-programmable mode.

ARDY - AREADY (Output):

A output handshaking line or data bit I/O line.

TPB (Input):

A positive input pulse used as a data load, set, or reset

strobe.

WR/RE - WRITE/READ ENABLE (Input):

A positive input used to write data from the CDP1851 to the

CPU bus.

RD/WE - READ/WRITE ENABLE (Input):

A positive input used to read data from the CPU bus to the

CDP1851 bus.

Positive supply voltage.

CDP1851, CDP1851C