Siemens EF 88H Series Manual de usuario descargar pdf (Pagina 46)

Semiconductor Group 6-12

On-Chip Peripheral Components

C501

The reason why read-modify-write instructions are directed to the latch rather than the pin is to avoid

a possible misinterpretation of the voltage level at the pin. For example, a port bit might be used to

drive the base of a transistor. When a “1” is written to the bit, the transistor is turned on. If the CPU

then reads the same port bit at the pin rather than the latch, it will read the base voltage of the

transistor (approx. 0.7 V, i.e. a logic low level!) and interpret it as “0”. For example, when modifying

a port bit by a SETB or CLR instruction, another bit in this port with the above mentioned

configuration might be changed if the value read from the pin were written back to the latch.

However, reading the latch rater than the pin will return the correct value of “1”.

Table 6-5

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

1 2 ... 41 42 43 44 45 46 47 48 49 50 51 ... 120 121

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Siemens EF 88H Series Manual de usuario Pagina 46