C501_1 Datasheet, PDF (42/121 Pages) Siemens Semiconductor Group – 8-Bit Single-Chip Microcontroller

On-Chip Peripheral Components
C501
6.1.2 Alternate Functions
The pins of ports 1 and 3 are multifunctional. They are port pins and also serve to implement special
features as listed in table 6-4.
Figure 6-9 shows a functional diagram of a port latch with alternate function. To pass the alternate
function to the output pin and vice versa, however, the gate between the latch and driver circuit must
be open. Thus, to use the alternate input or output functions, the corresponding bit latch in the port
SFR has to contain a one (1); otherwise the pulldown FET is on and the port pin is stuck at 0. After
reset all port latches contain ones (1).
Read
Latch
Int. Bus
Write
to
Latch
Alternate
Output
Function
Q
D
&
Bit
Latch
Q
CLK
V_CC
Internal
Pull Up
Arrangement
Pin
Read
Pin
Figure 6-9
Circuitry of Ports 1 and 3
Alternate
Input
Function
MCS01827
Semiconductor Group
6-8