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8XC251SA, SB, SP, SQ USER’S MANUAL
B-4
CEX2:0
CEX3
CEX4
I/O Programmable Counter Array (PCA) Input/Output Pins. These
are input signals for the PCA capture mode and output signals for
the PCA compare mode and PCA PWM mode.
P1.5:3
P1.6/WAIT#
P1.7/A17/WCLK
EA# I External Access. Directs program memory accesses to on-chip or
off-chip code memory. For EA# = 0, all program memory accesses
are off-chip. For EA# = 1, an access is to on-chip program memory
if the address is within the range of the on-chip program memory;
otherwise the access is off-chip. The value of EA# is latched at
reset. For devices without on-chip program memory, EA# must be
strapped to ground.
V
PP
ECI I PCA External Clock Input. External clock input to the 16-bit PCA
timer.
P1.2
INT1:0# I External Interrupts 0 and 1. These inputs set bits IE1:0 in the
TCON register. If bits IT1:0 in the TCON register are set, bits IE1:0
are set by a falling edge on INT1#/INT0#. If bits INT1:0 are clear,
bits IE1:0 are set by a low level on INT1:0#.
P3.3:2
P0.7:0 I/O Port 0. This is an 8-bit, open-drain, bidirectional I/O port. AD7:0
P1.0
P1.1
P1.2
P1.5:3
P1.6
P1.7
I/O Port 1. This is an 8-bit, bidirectional I/O port with internal pullups. T2
T2EX
ECI
CEX2:0
CEX3/WAIT#
CEX4/A17/WCLK
P2.7:0 I/O Port 2. This is an 8-bit, bidirectional I/O port with internal pullups. A15:8
P3.0
P3.1
P3.3:2
P3.5:4
P3.6
P3.7
I/O Port 3. This is an 8-bit, bidirectional I/O port with internal pullups. RXD
TXD
INT1:0#
T1:0
WR#
RD#/A16
PROG# I Programming Pulse. The programming pulse is applied to this pin
for programming the on-chip nonvolatile memory.
ALE
PSEN# O Program Store Enable. Read signal output to external memory.
Asserted for the address range specified by configuration byte
UCONFIG0, bits RD1:0 (Table B-3). Also see RD#.
—
RD# O Read. Read signal output to external data memory. Asserted for
the address range specified by configuration byte UCONFIG0, bits
RD1:0 (Table B-3). Also see PSEN# and A16.
P3.7/A16
Table B-2. Signal Descriptions (Continued)
Signal
Name
Type Description
Alternate
Function
†
The descriptions of A15:8/P2.7:0 and AD7:0/P0.7:0 are for the nonpage mode chip configuration (com-
patible with 44-pin PLCC and 40-pin DIP MCS
®
51 microcontrollers). If the chip is configured for page
mode operation, port 0 carries the lower address bits (A7:0), and port 2 carries the upper address bits
(A15:8) and the data (D7:0).