C541U_99 Datasheet, PDF (53/192 Pages) Siemens Semiconductor Group – 8-BIT CMOS MICROCONTROLLER

On-Chip Peripheral Components
C541U
6.1.1.3 Port 0 Circuitry
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 6-6) 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.
Read
Latch
Int. Bus
Write
to
Latch
Addr./Data
V_CC
Control
&
=1
Port
Pin
Q
D
Bit
Latch
Q
CLK
MUX
Read
Pin
Figure 6-6
Port 0 Circuitry
MCS02122
Semiconductor Group
6-8
1999-04-01