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8xC251Tx Hardware Description
Signal
Type
Name
P3.0
I/O
Port 3. This is an 8 bit, bidirectional I/O port with internal pullups
P3.1
P3.2
P3.3
P3.4
P3.5
P3.6
P3.7
PSEN#
O
Program Store Enable. Read signal output to external memory.
Asserted for the address range specified by the configuration byte
UCONFIG0, bits RD1:0.
RD#
O
Read. Read signal output to external memory. Asserted for the
address range specified by the configuration byte UCONFIG0, bits
RD1:0.
RST
I
Reset. Reset input to the chip. Holding this pin high for 64 oscillator
periods while the oscillator is running resets the device. The port pins
are driven to their reset conditions when a voltage greater than VIH1
is applied, whether or not the oscillator is running. This signal has a
Schmitt trigger input. Connecting the RST pin to V
capacitor provides power-on reset. Asserting RST when the chip is in
idle mode or powerdown mode returns the chip to normal operation.
RXD
I/O
Receive Serial Data. RXD send and receives data in serial I/O mode
0 and receives data in serial I/O modes 1, 2 and 3.
RXD1
I/O
Receive Serial Data 1. RXD send and receives data in serial I/O
mode 0 and receives data in serial I/O modes 1, 2 and 3 for the sec-
ond serial I/O port.
T1:0
I
Timer 1:0 External Clock Inputs. When Timer 1:0 operates as a
counter, a falling edge on the T1:0 pin increments the count.
T2
I/O
Timer 2 Clock Input/Output. For Timer 2 capture mode, this signal
is the external clock input. For the clock-out mode, it is the timer 2
clock input.
T2EX
I
Timer 2 External Input. In Timer 2 capture mode, a falling edge ini-
tiates a capture of Timer 2 registers. In auto-reload mode, a falling
edge causes the Timer 2 registers to be reloaded. In the up-down
counter mode, this signal determines the count direction:
• 1=up
• 0 = down.
TXD
O
Transmit Serial Data. TXD outputs the shift clock in serial I/O mode
0 and transmits serial data in serial I/O modes 1, 2 and 3.
TXD1
O
Transmit Serial Data 1. TXD outputs the shift clock in serial I/O
mode 0 and transmits serial data in serial I/O modes 1, 2 and 3 for the
second serial I/O port.
V
PWR
Supply Voltage. Connect this pin to the +5 supply voltage.
CC
* The descriptions of A15:8/P2.7:0 and AD7:0/P0.7:0 are for non page mode configuration. If configured in
page mode, Port 0 carries the lower address bits (A7:0) and Port 2 carries the upper address bits (A15:8)
and the data (D7:0)
Table 2. 8xC251Tx Signal Descriptions (Sheet 2 of 3)
Description
CC
4
Alternate
Function
RXD
TXD
INT0#
INT1#
T0
T1
WR#
RD#/A16
P3.7/A16
through a
P3.0
P1.2/ECI
P3.5:4
P1.0
P1.1
P3.1
P1.3/CEX0
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