Interfacing of Devices to Microcontroller

Interfacing of Devices to Microcontroller

Interfacing of Devices to Microcontroller

Prof Prabhat Ranjan

([email protected])

Dhirubhai Ambani Institute of Information and

Communication Technology, Gandhinagar


Based on a document “Microcontroller Interfacing

Techniques” available from



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


Digital I/O - On/OFF control and monitoring


Simplest interface

Lowest-cost to implement (built into the microcontroller)

High speed

Low programming overhead


Only on/off control/monitoring

Short distance, few feet maximum

Single device control/ monitoring

Digital Input Example: Reading the status of buttons or switches

Digital Input : Keypad

Digital Output : LED display

Digital Output : Relay Control

Analog I/O : Voltage-based control and monitoring

Simple interface

Low cost for lowresolutions

High speed

Low programming overhead

High cost for higher resolutions

Not all uC have analog

I/O built-in

Complicates the circuit design when external

ADC or DAC are needed

Short distance, few feet maximum

Analog I/O types

Voltage Ranges

0 to 2.5 V

0 to 4 V

0 to 5 V

+/- 2.5 V

+/- 4 V

+/- 5 V

Current Ranges

0-20 mA

4-20 mA

Analog Interface

Parallel Bus

Consists of multiple digital inputs/outputs. Most common types:

• 4-bit

• 8-bit ( e.g. Centronics )

• 16-bit ( e.g. ISA )

• 32-bit ( e.g. PCI )

High speed

High throughput:

Several bits are transmitted on one clock transition

Low cost

Large number of microcontroller pins needed for implementing the parallel bus

Parallel Example : 4 bit LCD interface

8 Bit LCD Interface

Serial Buses : I

Circuit bus)


C (Inter Integrated

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

Serial Buses : I

Circuit bus)


C (Inter Integrated

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, I


C is 5V

Multiple slave devices

- with only 3 wires

Low-cost to implement

Implemented in hardware or software

Ease to implement, many examples

Supports multi-master configuration

Short distance

Slow speed: 100 KHz although 400 KHz and

1 MHz slave device exist

These can not coexist with slower devices

Limited device addresses

Start & Stop

An I


C master prepares to communicate with a slave device first by generating a Start condition on the bus

Start condition is defined as SDA signal going low while SCL signal is high

Stop condition is defined as SDA going high while

SCL is high

Data Validity

Data can change while the clock is low

Data should remain stable while the clock is going high


Data transfer is initiated with the START bit (S) when SDA is pulled low while SCL stays high. Then, SDA sets the transferred bit while SCL is low (blue) and the data is sampled

(received) when SCL rises (green). When the transfer is complete, a STOP bit (P) is sent by releasing the data line to allow it to be pulled up while SCL is constantly high.

Writing a byte to a serial EEPROM (24C04 ) on the I2C bus:

● where Device Address is defined as : P0, P1, P2 indicate the page number ( 2Kbit pages ). A0, A1,

A2 indicate the device number on the bus

Reading a byte from a serial EEPROM (24C04 ) on the I2C bus ( starting from the current address )

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 ( Serial Peripheral Interface )

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

Multiple slave devices

- with only few wires


HW/SW Implement

Ease to implement, many examples

Can be high speed

( e.g. 4MHz or higher if implemented in hardware )

Short distance

Data and clock lines can be shared but each device requires a separate Chip Select signal, limiting the number of devices in limited I/O systems

SPI bus specifies four logic signals

SCLK — Serial Clock

(output from master)

MOSI — Master Output,

Slave Input (output from master)

MISO — Master Input,

Slave Output (output from slave)

SS — Slave Select (active low; output from master

Sometimes, the following naming convention is used:

SCLK — Serial Clock

(output from master)

SDI — Serial Data In

SDO — Serial Data Out

CS — Chip Select (active low; output from master)

SPI Bus with multiple slaves

Multimedia Card ( MMC ) Interface using SPI


Originated by Dallas Semiconductor ( now part of

MAXIM ) to address a variety of peripherals, sensors, and memory chips from a single wire interface ( DATA and Ground ).

One signal wire carries both operating power and signal. Usually the network is built using a wire pair where one wire carries the signal and power and the other wire is ground.

The system is sensitive to the right timing to operate well

Multiple slave devices- with only 2 wires


Implemented in HW/SW

Ease to implement, many examples

Relatively long distance.

Theoretically 300 meters but this is limited in practice due to noise and cable capacitance

Slow speed

1-wire slave devices typically has to come from one source:

Dallas Semiconductor


Asynchronous communications

Popular interface with many examples

Many compatible legacy devices

Relatively long distance, 50 feet maximum for low baud rates although longer distances work in practice, with low baud rates and error correction

Immune to noise due to +/-5 Volts or higher voltage levels for logic “0” and “1”

Implemented in hardware or software

Ease to implement, many examples

More suitable for system to system communications, not so much for chip to chip or chip to sensor

Low speed for long distance, 115200 baud can be achieved with small microcontrollers using short distances

Requires transceiver chips which add to system cost ( TTL/CMOS level RS232 can be used without transceiver chips ).

Single master/single slave


Asynchronous communications

Popular interface

Very long distance, thousands of feet

Immune to noise due to differential voltage

Implemented in hardware or software

Ease to implement

Widely used in industrial automation

Higher speeds beyond

115200 baud

More suitable for system to system communications, not so much for chip to chip or chip to sensor

Requires transceiver chips and twisted pair cable with terminating resistors which add to system cost

RS485 Network Topology: Any station can communicate with any other station, but not at the same time

UART : Universal asynchronous receiver/transmitter

A universal asynchronous receiver/transmitter

(UART) is a type of "asynchronous receiver/transmitter", a piece of computer hardware that translates data between parallel and serial interfaces

Used for serial data telecommunication, a UART converts bytes of data to and from asynchronous start-stop bit streams represented as binary electrical impulses

UART : Universal asynchronous receiver/transmitter

UARTs are commonly used in conjunction with other communication standards such as EIA


A UART is usually an individual (or part of an) integrated circuit used for serial communications over a computer or peripheral device serial port.

UARTs are now commonly included in microcontrollers. A dual UART or DUART combines two UARTs into a single chip.

As of now, UARTs are commonly used with

RS-232 for embedded systems communications.

It is useful to communicate between microcontrollers and also with PCs.

Many chips provide UART functionality in silicon, and low-cost chips exist to convert logic level signals (such as TTL voltages) to RS-232 level signals (for example, Maxim MAX232)


Very high speed (10

Mbit to 100 Mbit/s )

Very long distance, hundreds of feet can be achieved, more with hubs and switches

Immune to noise

Widely used in industrial automation due to noise immunity


More suitable for system to system communications, not so much for chip to chip/sensor

Requires Ethernet chipset, transformer, jack and special cabling that add to system cost.

Complicated to implement

High code footprint


Hardware interface

Software emulation : Bit banging

Bit banging

Bit-banging is a technique in embedded systems for example to use serial communications without the use of dedicated hardware such as a UART or shift register, instead using software to emulate their behavior

A software routine handles the UART transmit function by alternating a pin on the microcontroller by given time intervals

Bit banging

A receiver function is implemented by sampling a pin on the microcontroller by a given time interval

With a few extra components, video signals can be output from digital pins

Although it is not referred to as bit-banging, software-defined radio is an extreme extension of the same idea

Bit banging

Although it is often considered to be something of a hack, bit-banging does allow greater flexibility, allowing the same device to speak different protocols with minimal or no hardware changes required

There are some problems with bit-banging

More power is normally consumed in the software emulation process than in dedicated hardware

Bit banging

Another problem is that the microcontroller is busy most of the time looking at samples or sending a sample to the pin, instead of performing other tasks

Yet another potential problem is that the signal produced normally has more jitter or glitches, especially when the microcontroller moves on to perform other tasks.

However, if the bit-banging software is hardware interrupt-driven by the signal, this may be of minor importance

More Buses


RS232 <-> UART



Diff between TWI and I






Controller Area Network (CAN) is a broadcast, differential serial bus standard, originally developed in the 1980s by Robert Bosch GmbH, for connecting electronic control units (ECUs)

CAN was specifically designed to be robust in electromagnetically noisy environments and can utilize a differential balanced line like RS-485

CAN ...

It can be even more robust against noise if twisted pair wire is used.

Although initially created for automotive purposes

(as a vehicle bus), nowadays it is used in many embedded control applications (e.g., industrial) that may be subject to noise

The messages it sends are small (8 data bytes max) but are protected by a CRC-15 (polynomial

0x62CC) that guarantees a Hamming bit length of

6 (so up to 5 bits in a row corrupted will be detected by any node on the bus).

CAN ...

Bit rates up to 1 Mbit/s are possible at networks length below 40 m. Decreasing the bit rate allows longer network distances (e.g. 125 kbit/s at 500 m)


Universal Serial Bus (USB) is a serial bus standard to interface devices

It was originally designed for computers, but its popularity has prompted it to also become commonplace on video game consoles, PDAs, portable DVD and media players, cellphones; and even devices such as televisions, home stereo equipment, car stereos and portable memory devices

The radio spectrum based USB implementation is known as Wireless USB

USB : Overview

USB was devised as a major component in the transition towards a legacy-free PC

The intention was to let go of all older serial and parallel ports on personal computers since these were not properly standardized and required a multitude of device drivers to be developed and maintained

A USB system has an asymmetric design, consisting of a host controller and multiple daisychained devices

USB : Overview

Additional USB hubs may be included in the chain, allowing branching into a tree structure

No more than 127 devices, including the bus devices, may be connected to a single host controller

Modern computers often have several host controllers, allowing a very large number of USB devices to be connected

USB cables do not need to be terminated

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