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| 型号： | ST72121J2T6/XXX |
| 厂商： | STMICROELECTRONICS |
| 元件分类： | 微控制器/微处理器 |
| 英文描述： | 8-BIT, MROM, 16 MHz, MICROCONTROLLER, PQFP44 |
| 封装： | PLASTIC, TQFP-44 |
| 文件页数： | 16/87页 |
| 文件大小： | 1365K |
| 代理商： | ST72121J2T6/XXX |
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ST72121
4 ON-CHIP PERIPHERALS
4.1 I/O PORTS
4.1.1 Introduction
The I/O ports offer different functional modes:
– transfer of data through digital inputs and outputs
and for specific pins:
– analog signal input (ADC)
– alternate signal input/output for the on-chip pe-
ripherals.
– external interrupt generation
An I/O port is composed of up to 8 pins. Each pin
can be programmed independently as digital input
(with or without interrupt generation) or digital out-
put.
4.1.2 Functional Description
Each port is associated to 2 main registers:
– Data Register (DR)
– Data Direction Register (DDR)
and some of them to an optional register:
– Option Register (OR)
Each I/O pin may be programmed using the corre-
sponding register bits in DDR and OR registers: bit
X corresponding to pin X of the port. The same cor-
respondence is used for the DR register.
The following description takes into account the
OR register, for specific ports which do not provide
this register refer to the I/O Port Implementation
Section 4.1.2.5. The generic I/O block diagram is
shown on Figure 18.
4.1.2.1 Input Modes
The input configuration is selected by clearing the
corresponding DDR register bit.
In this case, reading the DR register returns the
digital value applied to the external I/O pin.
Different input modes can be selected by software
through the OR register.
Notes:
1. All the inputs are triggered by a Schmitt trigger.
2. When switching from input mode to output
mode, the DR register should be written first to
output the correct value as soon as the port is con-
figured as an output.
Interrupt function
When an I/O is configured in Input with Interrupt,
an event on this I/O can generate an external In-
terrupt request to the CPU. The interrupt polarity is
given independently according to the description
mentioned in the Miscellaneous register or in the
interrupt register (where available).
Each pin can independently generate an Interrupt
request.
Each external interrupt vector is linked to a dedi-
cated group of I/O port pins (see Interrupts sec-
tion). If more than one input pin is selected simul-
taneously as interrupt source, this is logically
ORed. For this reason if one of the interrupt pins is
tied low, it masks the other ones.
4.1.2.2 Output Mode
The pin is configured in output mode by setting the
corresponding DDR register bit.
In this mode, writing “0” or “1” to the DR register
applies this digital value to the I/O pin through the
latch. Then reading the DR register returns the
previously stored value.
Note: In this mode, the interrupt function is disa-
bled.
4.1.2.3 Digital Alternate Function
When an on-chip peripheral is configured to use a
pin, the alternate function is automatically select-
ed. This alternate function takes priority over
standard I/O programming. When the signal is
coming from an on-chip peripheral, the I/O pin is
automatically configured in output mode (push-pull
or open drain according to the peripheral).
When the signal is going to an on-chip peripheral,
the I/O pin has to be configured in input mode. In
this case, the pin’s state is also digitally readable
by addressing the DR register.
Notes:
1. Input pull-up configuration can cause an unex-
pected value at the input of the alternate peripher-
al input.
2. When the on-chip peripheral uses a pin as input
and output, this pin must be configured as an input
(DDR = 0).
Warning: The alternate function must not be acti-
vated as long as the pin is configured as input with
interrupt, in order to avoid generating spurious in-
terrupts.
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