ST62T52C datasheet(38/78 Pages) STMICROELECTRONICS

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ST62T52C ST62T62C/E62C

I/O PORTS (Cont’d)

4.1.2 Safe I/O State Switching Sequence

Switching the I/O ports from one state to another

should be done in a sequence which ensures that

no unwanted side effects can occur. The recom-

mended safe transitions are illustrated in Figure

23.. All other transitions are potentially risky and

should be avoided when changing the I/O operat-

ing mode, as it is most likely that undesirable side-

effects will be experienced, such as spurious inter-

rupt generation or two pins shorted together by the

analog multiplexer.

Single bit instructions (SET, RES, INC and DEC)

should be used with great caution on Ports Data

registers, since these instructions make an implicit

read and write back of the entire register. In port

input mode, however, the data register reads from

the input pins directly, and not from the data regis-

ter latches. Since data register information in input

mode is used to set the characteristics of the input

pin (interrupt, pull-up, analog input), these may be

unintentionally reprogrammed depending on the

state of the input pins. As a general rule, it is better

to limit the use of single bit instructions on data

registers to when the whole (8-bit) port is in output

mode. In the case of inputs or of mixed inputs and

outputs, it is advisable to keep a copy of the data

be used on the RAM copy, after which the whole

copy register can be written to the port data regis-

ter:

SET bit, datacopy

LD a, datacopy

LD DRA, a

Warning: Care must also be taken to not use in-

structions that act on a whole port register (INC,

DEC, or read operations) when all 8 bits are not

available on the device. Unavailable bits must be

masked by software (AND instruction).

The WAIT and STOP instructions allow the

ST62xx to be used in situations where low power

consumption is needed. The lowest power con-

sumption is achieved by configuring I/Os in input

mode with well-defined logic levels.

The user must take care not to switch outputs with

heavy loads during the conversion of one of the

analog inputs in order to avoid any disturbance to

the conversion.

Figure 23. Diagram showing Safe I/O State Transitions

Note *. xxx = DDR, OR, DR Bits respectively

Interrupt

pull-up

Output

Open Drain

Output

Push-pull

Input

pull-up (Reset

state)

Input

Analog

Output

Open Drain

Output

Push-pull

Input

010*

000

100

110

011

001

101

111

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