C509-L_97 SIEMENS [Siemens Semiconductor Group], C509-L_97 Datasheet - Page 86

• Manufacturer Part Number: C509-L_97
• Description: 8-Bit CMOS Microcontroller
• Manufacturer: SIEMENS [Siemens Semiconductor Group]

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.
Figure 6-5
Port 0 Circuitry
Semiconductor Group
Read
Int.
Read
Pin
Latch
Write to
Latch
Bus
D
CLK
Bit
Latch
Q
Q
6-8
Addr./Data
=1
On-Chip Peripheral Components
MUX
Control
&
MCS02434
V
CC
Port
Pin
1997-10-01
C509-L