C508_01 Datasheet, PDF (71/267 Pages) Infineon Technologies AG – 8-Bit CMOS Microcontroller

C508
On-Chip Peripheral Components
6.1.2.1 Port 0 Circuitry
Port 0, in contrast to Ports 1 to 5, is considered a “true” bidirectional port because its pins
float when configured as inputs. Thus, this port differs in not having internal pull-ups. The
pull-up 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 pull-up 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 this condition, it can
be used as high-impedance input. If Port 0 is configured as a general I/O port and must
emit logic high-level (1); then, external pull-ups are required.
Read
Latch
Int. Bus
Write
to
Latch
Addr./
Data Control
V_DD
&
D
Q
Bit
Latch
CLK
Q
Port
Pin
MUX
Read
Pin
Figure 6-3 Port 0 Circuitry
MCS04043
User’s Manual
6-6
2001-05