EF6805P2 datasheet(16/31 Pages) STMICROELECTRONICS

16 / 31 page

A software interrupt (SWI) is an executable instruc-

tion which is executed regardless of the state of the

I bit in the condition code register. SWIs are usually

used as break-points for debugging or a system

calls.

Figure 19 : Typical Interrupt Circuits.

INPUT/OUTPUT CIRCUITRY

There are 32 input/output pins. The INT pin may be

polled with branch instructions to provide an additio-

nal input pin. All pins on ports A, B, and C are pro-

grammable as either inputs or outputs under soft-

ware control of the corresponding data direction re-

gister (DDR). See below I/O port control registers

configuration. The port I/O programming is ac-

complished by writing the corresponding bit in the

port DDR to a logic one for output or a logic zero for

input. On reset all the DDRs are initialized to a logic

zero state, placing the ports in the input mode. The

port output registers are not initialized on reset and

should be initialized by software before changing the

DDRs from input to output. A read operation on a

port programmed as an output will read the contents

of the output latch regardless of the logic levels at

the output pin, due to output loading. Refer to figure

20.

PORT DATA REGISTER

Port A Addr = $000

Port B Addr = $001

Port C Addr = $002

Port D Addr = $003

PORT DATA DIRECTION REGISTER (DDR)

(1) Write only ; reads as all “1s”

(2) 1 = Output.0 = input Cleared to 0 by Reset

(3) Port A Addr = $004

Port B Addr = $005

Port C Addr = $006

EF6805U3

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