C511_1 Datasheet, PDF (35/128 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

On-Chip Peripheral Units
Port 1, 2 and 3 output drivers have internal pullup FET’s (see figure 6-6-14). 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 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 pullups, 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 6-14
Basic Output Driver Circuit of Ports 1, 2 and 3
Semiconductor Group
6-2