C505_9708 SIEMENS [Siemens Semiconductor Group], C505_9708 Datasheet - Page 67

• Manufacturer Part Number: C505_9708
• Description: 8-Bit CMOS Microcontroller
• Manufacturer: SIEMENS [Siemens Semiconductor Group]

6.1.2.1
Port 0, in contrast to ports 1 to 4, 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 6-3) 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 6-3
Port 0 Circuitry
Semiconductor Group
Port 0 Circuitry
Read
Int.
Read
Pin
Latch
Write to
Latch
Bus
D
CLK
Bit
Latch
Q
Q
6-5
Addr./Data
=1
On-Chip Peripheral Components
MUX
Control
&
MCS02434
V
CC
C505 / C505C
Port
Pin
1997-08-01