C509-L_97 Datasheet, PDF (86/290 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

On-Chip Peripheral Components
C509-L
6.1.1.2.2 Port 0 Circuitry
Port 0, in contrast to ports 1 to 6 and 9, 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 6-5) 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
Addr./Data
V_CC
Control
&
=1
Port
Pin
Int. Bus
D
Q
Bit
Latch
Write to
Latch
CLK
Q
MUX
Read
Pin
Figure 6-5
Port 0 Circuitry
MCS02434
Semiconductor Group
6-8
1997-10-01