Intel 8XC251SP User Manual

7-1

The 8XC251Sx uses input/output (I/O) ports to exchange data with external devices. In addition
to performing general-purpose I/O, some ports are capable of external memory operations (see
Chapter 13, “External Memory Interface”); others allow for alternate functions. All four
8XC251Sx I/O ports are bidirectional. Each port contains a latch, an output driver, and an input
buffer. Port 0 and port 2 output drivers and input buffers facilitate external memory operations.
Port 0 drives the lower address byte onto the parallel address bus, and port 2 drives the upper ad-
dress byte onto the bus. In nonpage mode, the data is multiplexed with the lower address byte on
port 0. In page mode, the data is multiplexed with the upper address byte on port 2. All port 1 and
port 3 pins serve for both general-purpose I/O and alternate functions (see Table 7-1). 

P0.7:0

I/O

AD7:0

Address/Data (Nonpage Mode), Address (Page Mode)

I/O

P1.0

I/O

T2

Timer 2 Clock Input/Output

I/O

P1.1

I/O

T2EX

Timer 2 External Input 

I

P1.2

I/O

ECI

PCA External Clock Input

I

P1.3

I/O

CEX0

PCA Module 0 I/O

I/O

P1.4

I/O

CEX1

PCA Module 1 I/O

I/O

P1.5

I/O

CEX2

PCA Module 2 I/O

I/O

P1.6

I/O

CEX3/WAIT#

PCA Module 3 I/O

I/O

P1.7

I/O

CEX4/A17/WCLK PCA Module 4 I/O or 18th Address Bit

I/O(O)

P2.7:0

I/O

A15:8

Address (Nonpage Mode), Address/Data (Page Mode)

I/O

P3.0

I/O

RXD

Serial Port Receive Data Input

I (I/O)

P3.1

I/O

TXD

Serial Port Transmit Data Output

O (O)

P3.2

I/O

INT0#

External Interrupt 0 

I

P3.3

I/O

INT1#

External Interrupt 1 

I

P3.4

I/O

T0

Timer 0 Input

I

P3.5

I/O

T1

Timer 1 Input

I

P3.6

I/O

WR#

Write Signal to External Memory

O

P3.7

I/O

RD#/A16

Read Signal to External Memory or 17th Address Bit

O