C511 datasheet(46/128 Pages) SIEMENS

46 / 128 page

On-Chip Peripheral Units

Semiconductor Group

6-13

6.1.4.2 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 be seen in the Data Sheet of the SAB-C511/513

family members. The corresponding DC parameters are

OL and VOH.

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

TL and IIL in the DC characteristics specify

these currents). Port 0 has floating inputs when used for digital input.

6.1.4.3

Read-Modify-Write Feature of Ports 1, 2 and 3

Some port-reading instructions read the latch and others read the pin (see figure 6-13). 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 6-4. 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, resp., is performed by reading the SFR P0, P1, P2 and P3; for example,

"MOV A, P1" reads the value from port 3 pins, while "ANL P1, #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 6-4 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.

Table 6-4

"Read-Modify-Write"-Instructions

Instruction

Function

ANL

Logic AND; e.g. ANL P1, A

ORL

Logic OR; e.g. ORL P2, A

XRL

Logic exclusive OR; e.g. XRL P3, A

JBC

Jump if bit is set and clear bit; e.g. JBC P1.1, LABEL

CPL

Complement bit; e.g. CPL P3.0

INC

Increment byte; e.g. INC P1

DEC

Decrement byte; e.g. DEC P1

DJNZ

Decrement and jump if not zero; e.g. DJNZ P3, LABEL

MOV Px.y,C

Move carry bit to bit y of port x

CLR Px.y

Clear bit y of port x

SETB Px.y

Set bit y of port x

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