C501_1 Datasheet, PDF (45/121 Pages) Siemens Semiconductor Group – 8-Bit Single-Chip Microcontroller

On-Chip Peripheral Components
C501
6.1.3.2 Port Loading and Interfacing
The output buffers of ports 1, 2 and 3 can drive TTL inputs directly. The maximum port load which
still guarantees correct logic output levels can be looked up in the C501 DC characteristics in
chapter 10. The corresponding parameters are V_OL and V_OH.
The same applies to port 0 output buffers. They do, however, require external pullups to drive
floating inputs, except when being used as the address/data bus.
When used as inputs it must be noted that the ports 1, 2 and 3 are not floating but have internal
pullup transistors. The driving devices must be capable of sinking a sufficient current if a logic low
level shall be applied to the port pin (the parameters I_TL and I_IL in the C501 DC characteristics
specify these currents). Port 0 has floating inputs when used for digital input.
6.1.3.3 Read-Modify-Write Feature of Ports 1,2 and 3
Some port-reading instructions read the latch and others read the pin. The instructions reading the
latch rather than the pin read a value, possibly change it, and then rewrite it to the latch. These are
called “read-modify-write”- instructions, which are listed in table 6-5. If the destination is a port or a
port pin, these instructions read the latch rather than the pin. Note that all other instructions which
can be used to read a port, exclusively read the port pin. In any case, reading from latch or pin,
respectively, is performed by reading the SFR P0, P1, P2 and P3; for example, “MOV A, P3” reads
the value from port 3 pins, while “ANL P3, #0AAH” reads from the latch, modifies the value and
writes it back to the latch.
It is not obvious that the last three instructions in table 6-5 are read-modify-write instructions, but
they are. The reason is that they read the port byte, all 8 bits, modify the addressed bit, then write
the complete byte back to the latch.
Semiconductor Group
6-11