SAB80C517-M16 SIEMENS [Siemens Semiconductor Group], SAB80C517-M16 Datasheet - Page 48

• Manufacturer Part Number: SAB80C517-M16
• Description: 8-Bit CMOS Single-Chip Microcontroller
• Manufacturer: SIEMENS [Siemens Semiconductor Group]

On-Chip Peripheral Components
7.1.4.2 Port Loading and Interfacing
The output buffers of ports 1 through 6 can drive TTL inputs directly. The maximum port load which
still guarantees correct logic output levels can be looked up in the DC characteristics in the Data
Sheet of the SAB 80C517. The corresponding parameters are
V
and
V
.
OL
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 through 6 are not floating but have internal
pullup transistors. The driving devices must be capable of sinking a sufficient current if a logic low
I
I
level shall be applied to the port pin (the parameters
and
in the DC characteristics specify
TL
IL
these currents). Port 0 as well as the input only ports 7 and 8, however, have floating inputs when
used for digital input.
7.1.4.3 Read-Modify-Write Feature of Ports 0 through 6
Some port-reading instructions read the latch and others read the pin (see figure 7-1). 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 7-2. lf the
destination is a port or a port bit, 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, resp., is performed by reading the SFR P0 to P6; for example,
"MOV A, P3" reads the value from port 3 pins, while "ANL P4, #0AAH" reads from the latch,
modifies the value and writes it back to the latch.
lt is not obvious that the last three instructions in this list 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
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