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ST7285C Datasheet(HTML) 26 Page - STMicroelectronics

Part No. ST7285C
Description  8-BIT MCU FOR RDS WITH 48K ROM, 3K RAM, ADC, TWO TIMERS, TWO SPIs, I2C AND SCI INTERFACES
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Maker  STMICROELECTRONICS [STMicroelectronics]
Homepage  http://www.st.com
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ST7285C Datasheet(HTML) 26 Page - STMicroelectronics

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ST7285C
I/O PORTS (Cont’d)
4.1.2.3 Operating Modes
All I/O pins may be configured as inputs or outputs
by programming the corresponding bits of the DR,
DDR, OR and PUR memory-mapped registers.
Table 6 illustrates the available operating modes.
During Reset, DR, DDR, OR and PUR are initial-
ized to a Low level.
Table 6. I/O Operating Modes
Note: (1) This state can add static current consumption.
– Input Mode
In input mode, both the analog multiplexer and the
port buffer are switched to a high impedance state.
To avoid ringing with slowly rising or falling input
signals and to increase noise immunity, the inputs
are equipped with Schmitt-triggers.
The state of the pin is readable through the Data
Register. The pin state is read directly from the
Schmitt Trigger’s output and not from the Data
Register.
There are four different input modes, as illustrated
in Table 6.
Note: Pull-up and pull-down devices are not imple-
mented by means of linear resistors, but by means
of resistive transistors.
– Interrupt function
The interrupt signals of all activated bits are
NANDed together, so that whenever at least one
of the activated inputs goes low, the port’s com-
mon interrupt output will go high in order to acti-
vate the CPU interrupt input.
– Output Mode
In output mode, the port output buffer is activated
and drives the output according to the content of
the data register, DR. In this mode, the analog
multiplexer, when present, is switched to high im-
pedance and the interrupt is disabled.
Data written to the DR is directly copied to the out-
put pins. A read operation of DR will be directly
performed from the DR register, so that the output
data stored in DR is readable, regardless of the
logic levels at the output pin due to output loading.
There are three different output modes for the
standard I/O pins as illustrated inTable 6.
– Alternate function
Alternate functions take priority over standard I/O
programming; if a peripheral needs to use a pad,
the alternate function is automatically activated.
The signal from the peripheral is output to the pad
(automatically configured in this case in push-pull
or open drain modes without pull-up and pull-
down), and controlled directly by the peripheral.
The signal to be input to the peripheral from the
pad is taken after the schmitt trigger and is control-
led directly by the peripheral. In this case, the pin’s
state is readable as in Input Mode by addressing
the Data Register and by configuring the PAD in
Input Mode (DDR=0).
– Analog Input Mode
In analog input mode (activated by the ADC), the
analog multiplexer is activated and switches the
analog voltage present on the selected pin (pins
PA0 to PA7) to the common analog rail. The com-
mon analog rail is connected to the Analog to Dig-
ital converter (see Section 4.6) input. It is not rec-
ommended to change the voltage level or loading
on any port pin while conversion is in progress.
Furthermore it is not recommended to have clock-
ing pins located close to a selected analog pad.
WARNING: Before activating the Analog Input
Mode, the I/O state must be set to:
INPUT, NO PULL-UP, NO INTERRUPT
(DDR = 0, OR = 0, PUR = 1)
The alternate function must not be activated as
long as the pad is configured as Input with Inter-
rupt, in order to avoid generating spurious inter-
rupts.
Analog input mode is only implemented for pins
PA0 to PA7. The analog input voltage level must
be within the limits stated in the Absolute Maxi-
mum Ratings.
DDR
OR
PUR
Mode
Option
0
0
0
input
pull-up, no interrupt
0
0
1
input
no pull-up, no interrupt
0
1
0
input
pull-up, interrupt
0
1
1
input
pull-down, no interrupt
1
0
0
output
open-drain, pull-up
1
0
1
output
open-drain, no pull-up
1
1
0
output
RESERVED (1)
1
1
1
output
push-pull, no pull-up, no
pull-down


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