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SECTION 8
INPUT/OUTPUT CIRCUITRY AND
ANALOG-TO-DIGITAL CONVERTER
8.1 INPUT/OUTPUT CIRCUITRY
There are 32 input/ output pins. The INT pin may be polled with branch instructions to provide an
additional input pin. All pins on ports A, B, and C are programmable as either inputs or outputs
under software control of the corresponding data direction register (DDR)' The port I/O programm-
ing is accomplished by writing the corresponding bit in the port DDR to a logic one for output or a
logic zero for input. On reset all the DDRs are initialized to a logic zero state, placing the ports in the
input mode. The port output registers are not initialized on reset and should be initialized by soft-
ware before changing the DDRs from input to output. A read operation on a port programmed as
an output will read the contents of the output latch regardless of the logic levels at the output pin,
due to output loading. Refer to Figure 8-1.
Internal
Connections
*
DDR is a write-only register and reads as all ones.
Data
Direction
Register
Bit
, ,
0
Latched
Output
Data
Bit
0
,
X
Input
Output
To
State
MCU
0
0
,
,
Hi~Z*
*
Pin
* *
Port Band C are three-state ports. Port A has optional internal pullup devices to provide CMOS data drive capability.
See SECTION 11 ELECTRICAL CHARACTERISTICS.
Figu re 8-1. Typical Port I/O Circu itry
8-1
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