C501_1 Datasheet, PDF (37/121 Pages) Siemens Semiconductor Group – 8-Bit Single-Chip Microcontroller

On-Chip Peripheral Components
C501
Port 1, 2 and 3 output drivers have internal pullup FET’s (see figure 6-5). Each I/O line can be used
independently as an input or output. To be used as an input, the port bit stored in the bit latch must
contain a one (1) (that means for figure 6-5: Q=0), which turns off the output driver FET n1. Then,
for ports 1, 2 and 3, the pin is pulled high by the internal pullups, but can be pulled low by an external
source. When externally pulled low the port pins source current (I_IL or I_TL). For this reason these
ports are sometimes called “quasi-bidirectional”.
Read
Latch
Int. Bus
Write
to
Latch
Q
D
Bit
Latch
Q
CLK
V_CC
Internal
Pull Up
Arrangement
Pin
n1
Read
Pin
Figure 6-5
Basic Output Driver Circuit of Ports 1, 2, and 3
MCS01823
Semiconductor Group
6-3