C511_1 Datasheet, PDF (40/128 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

On-Chip Peripheral Units
Port 0, in contrast to ports 1, 2 and 3, is considered as "true" bidirectional, because the port 0 pins
float when configured as inputs. Thus, this port differs in not having internal pullups. The pullup FET
in the P0 output driver (see figure 6-16a) is used only when the port is emitting 1 s during the
external memory accesses. Otherwise, the pullup is always off. Consequently, P0 lines that are
used as output port lines are open drain lines. Writing a "1" to the port latch leaves both output FETs
off and the pin floats. In that condition it can be used as high-impedance input. If port 0 is configured
as general I/O port and has to emit logic high-level (1), external pullups are required.
Figure 6-16a
Port 0 Circuitry
Semiconductor Group
6-7