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On-Chip Peripheral Components
Port 1 through 5 output drivers have internal pullup FET's (see figure 7-2). 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 one
(1) (that means for figure 7-2: Q = 0), which turns off the output driver FET n1. Then, for ports 1
through 5, the pin is pulled high by the internal pullups, but can be pulled low by an external source.
When externally pulled low the port pins source current (
I
or
I
). For this reason these ports are
IL
TL
sometimes called "quasi-bidirectional".
*
Figure 7-2
Basic Output Driver Circuit of Ports 1 through 5
In fact, the pullups mentioned before and included in figure 7-2 are pullup arrangements as shown
in figure 7-3. These pullup arrangements are realized differently in the MYMOS and ACMOS
versions. In the next two sections both versions are discussed separately.
Semiconductor Group
37
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