Micro-controllers
are useful to the extent that they communicate with other devices, such as sensors, motors, switches, keypads, displays,
memory and even other micro-controllers. Many interface methods have been developed
over the years to solve the complex problem of balancing circuit design criteria such as
features, cost, size, weight, power consumption, reliability, availability,
manufacturability.
Many
microcontroller designs typically mix multiple interfacing methods. In a very simplistic form, a micro-controller system can be viewed
as a system that reads from (monitors) inputs, performs processing and writes to ( controls )
outputs.
Serial Buses
I2C ( Inter
Integrated Circuit bus )
2-wire
interface with one master and multiple slaves ( multi-master configurations
possible ).
Originated
by Philips Semiconductor in the early 80’s to connect a microcontroller to
peripheral devices in TV sets.
Signals:
DATA (SDA), CLOCK (SCL) and Ground. SDA is always bi-directional; SCL is
bidirectional only in multi-master mode.
Maximum
allowable capacitance on the lines is 400 pF. Typical device capacitance is 10
pF. To start the communications, the bus
master (typically a microcontroller) places the address of the device with
which it intends to communicate (the slave) on the bus. All slave devices
monitor the bus to determine if the master device is sending their address.
Only the
device with the correct address communicates with the master.
By
definition, I2C is 5V.
SPI ( Serial
Peripheral Interface )
4-wire
interface with one master and multiple slaves. Signals: DATA IN, DATA OUT,
CLOCK, CS
( Chip
Select )
Originated
by Motorola, SPI bus is a relatively simple synchronous serial interface for
connecting low speed external devices
using minimal number of wires. A synchronous clock shifts serial data into and
out of the microcontrollers in blocks of 8 bits.
SPI bus is a
master/slave interface. Whenever two devices communicate, one is referred to as
the "master" and the other as the "slave" device. The
master drives the serial clock. SPI is full duplex: Data is simultaneously
transmitted and received.
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