Peripheral
Modules
A microcontroller
seen as a whole represents a structure composed of a microprocessor and a set
of peripheral modules. A microcontroller can have a range of peripheral modules
depending on its series. A peripheral module as a rule is a specialized circuit
that can perform a certain function. See above some function examples:
• GPIO allows
setting or collection of logic level on a microcontroller pin
• EEPROM allows
long-term data storage.
• ADC allows
conversion of analog signals
• Serial UART
allows serial communication
• TIMER offers the
possibility to generate time intervals, measure the time between two events,
generate digital signals of certain forms.
In a certain case we could say that peripherals represent
the microcontroller interface with the external world.
The microprocessor (CPU) interaction with peripherals is
performed via peripheral registers. The AVR architecture has 64 registers,
called peripheral registers or I/O registers.
Each
peripheral module has in the peripheral registers space a reserved set of
registers. For example, each GPIO module has reserved a set of 3 registers
called PORT, PIN and DDR. By use of these registers the microprocessor is able
to work with the pin logic levels. We could do a classification of peripheral
registers depending on their destination as follows:
• Status Registers - offer the possibility to visualize the current state
of a peripheral (flags). As a rule status registers are available only for
reading.
• Control Registers - allow configuration of the peripheral module. They
are available for both writing and reading.
• Data Registers - enable data exchange between the peripheral module and kernel.
• Selection Registers - allow selection of resources / configurations in a
peripheral module.
In many cases peripheral registers
don't have a well-defined purpose, ie a register might contain a collection of
status bits, selection control. The purpose of peripheral registers can be
recognized by their name and often by their termination: _SR, _CR, _DR, _AR,
however it's not a rule.
The microprocessor (CPU) doesn't
distinguish the peripheral modules. It "sees" the peripheral modules
through peripheral registers and the the work with the peripherals occurs
through data transfer with an address from the peripheral registers space. The
access to peripheral registers will be performed by a special set of commands.
Transfer commands.
These commands involve transfers on 8 bits:
in R, P - content transfer of a
peripheral register to a general purpose register.
out P, R - content transfer of a general
purpose register to a peripheral register.
The commands of
setting/resetting of unique bits in a
peripheral register will affect only the bit specified by n, the
remaining bits will keep the value they had before the operation:
cbi P, n - assigning logical
"0", resetting the bit n of port P n = 0 .. 7
sbi P, n assigning logical
"1", setting the bit n of port P n = 0 .. 7
Verification /
testing commands of status bits within a peripheral register, ignoring the next
operation if the check is true:
sbic P, n - ignoring the following
command in case the bit in the port P is "0", reset.
sbis P, n - ignoring
the following command in case the bit in the port P is "1", set.
Before you start working with a specific peripheral
in most cases will be required its configuration / initialization. The process
of initialization will be performed
through setting the configuration to certain control peripheral registers. In some cases the
configuration after reset can be ok, but it is recommended each peripheral
module to be initialized on the entire set of configuration bits. The
configurations of each peripheral module can be found in the technical
documentation of the corresponding peripheral module.
In conclusion we
can affirm that peripheral modules represent electrical devices attached to
processor with access to peripheral
registers.
In order to
understand better what a peripheral module is, we could imagine a complex
panel, for example inside an airplane. What is seen on this panel are many
light bulbs, switches, indicators, etc. Each bulb can be associated with a
status bit, a switch with a configuration bit, an alphanumeric indicator -
output data etc.
Peripheral modules
can generate interrupts in case of appearance of special situations detected by
peripheral, such as changing peripheral logic level on the pin, completion of
data transfer, conversion completion etc. But we will talk about interruptions
later on.
GPIO Peripheral Module - Generic I/O Port.
Any microcontroller has a set of pins, most of which
can be configured as generic input/output (GPIO) pins so that it will be
possible to assign a logical value to
the terminal pin or this logical value to be read from it in case the
pin is configured to the input.
The input/output pins are grouped into ports by 8. The work
with the port is executed as a whole, therefore, the single transfer operation
to port can change at once the configuration of all 8 pins. In result, the
transfer operations to port will affect all its pins.
Since GPIO module is a peripheral
module of the microcontroller it has reserved a set of registers in the address
space of peripheral registers.
Each
GPIO module of a microcontroller of AVR architecture has 3 peripheral registers:
PINx, DDRx and PORTx.
The
explanation of how does the GPIO module work is presented by the following
figure:
• PINx - used for
reading the logical value of the physical terminal. This register has read only
access. The write operation to this register will not affect the physical value
of the physical terminal.
• PORTx - register for
setting a value of port when it is set to output and activating the pull-up
resistance in case when it is set to input.
• DDRx - register for
setting the direction of port, input or output.
In case the location of DDRx contains
"1", the corresponding pin will be set to output and the logical
value from PORTx from the same location will be transfered towards physical
terminal. In this case the value of PORTx register will conect the pull-up
resistance with logical value "1", the logical value "0"
will set the pin in the high impedance state - "HiZ".
It is recommended to avoid the HiZ on
terminal, fact that occurs when the terminal isn't conected to a signal source
in order to avoid the noise appearance inside the microcontroller chip that in
critical cases can result in circuit failure.
In this way the recommendation for the unused pins is to
set their configuration as input with activated pull-up resistance - action
which will avoid accidental short circuits when it is set as output and avoid
noise appearance on unwanted inputs when it is set to input.
