st62t62c STMicroelectronics, st62t62c Datasheet - Page 38

• Manufacturer Part Number: st62t62c
• Description: 8-bit Otp/eprom Mcus With A/d Converter, Safe Reset, Auto-reload Timer And Eeprom
• Manufacturer: STMicroelectronics

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
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
Figure 23. Diagram showing Safe I/O State Transitions
Note *. xxx = DDR, OR, DR Bits respectively
38/78
Interrupt
pull-up
Input
pull-up (Reset
state)
Output
Open Drain
Output
Push-pull
010*
000
100
110
Figure
outputs, it is advisable to keep a copy of the data
register in RAM. Single bit instructions may then
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.
011
001
101
111
Open Drain
Push-pull
Analog
Output
Output
Input
Input