C517A_99 Datasheet, PDF (71/218 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

On-Chip Peripheral Components
C517A
6.1.2.1 Port 0 Circuitry
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.
Read
Latch
Int. Bus
Write to
Latch
D
Q
Bit
Latch
CLK
Q
Addr./Data
Control
V^V_DCDC
&
=1
Port
Pin
MUX
Read
Pin
Figure 6-3
Port 0 Circuitry
MCS02434
Semiconductor Group
6-5