C509-L_97 Datasheet, PDF (83/290 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

On-Chip Peripheral Components
C509-L
6.1.1.2 Quasi-Bidirectional Port Structure
6.1.1.2.1 Basic Port Circuitry of Port 1 to 6 and 9
The basic quasi-bidirectional port structure as shown in the upper part of the schematics of
figure 6-1 provides a port driver circuit which is build up by an internal pullup FET as shown in
figure 6-3. 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 6-3: Q=0), which
turns off the output driver FET n1. Then, for ports 1 to 6 and 9, 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 6-3
Basic Output Driver Circuit of Ports 1 to 6 and 9
MCS01823
Semiconductor Group
6-5
1997-10-01