On-Chip Peripheral Components
C513AO
User’s Manual
6-2
05.99
Ports 1, 2, and 3 output drivers have internal pull-up FETs (see Figure 6-2). Each I/O line can be
used independently as an input or output. To be used as an input, the port bit must contain a 1 (that
means for Figure 6-2: Q = 0), which turns off the output driver FET n1. Then, for Ports 1, 2, and 3,
the pin is pulled high by the internal pull-ups, but can be pulled low by an external source. When
externally pulled low, the port pins source current (
I
IL
or
I
TL
). For this reason, these ports are
sometimes called “quasi-bidirectional”.
Figure 6-2
Basic Output Driver Circuit of Ports 1, 2, and 3
In fact, the pull-ups mentioned before, and included in Figure 6-2, are pull-up arrangements shown
in Figure 6-3.
MCS02431
D
CLK
Bit
Latch
Internal
Pull Up
Arrangement
V
DD
n1
Int. Bus
Write to
Pin
Read
Latch
Latch
Read
Port
Pin
Q
Q