Interfacing mc

8/11/2019 Interfacing mc

1/57

Interfacing of Devices to Microcontroller

Prof Prabhat Ranjan([email protected])

Dhirubhai Ambani Institute of Information andCommunication Technology, Gandhinagar


2/57

Acknowledgment

Based on a document MicrocontrollerInterfacing Techniques available fromwww.bipom.com

Wikipedia


3/57

Overview

Micro-controllers are useful to the extent thatthey communicate with other devices, such assensors, motors, switches, keypads, displays,memory and even other micro-controllers

Many interface methods have been developedover the years to solve the complex problem

of balancing circuit design criteria such asfeatures, cost, size, weight, powerconsumption, reliability, availability,manufacturability


4/57

Overview

Many microcontroller designs typically mixmultiple interfacing methods. In a verysimplistic form, a micro-controller system canbe viewed as a system that reads from(monitors) inputs, performs processing andwrites to ( controls ) outputs


5/57


6/57


7/57

Digital I/O - On/OFF control andmonitoring

Advantages Simplest interface Lowest-cost to

implement (built intothe microcontroller)

High speed

Low programmingoverhead

Disadvantages Only on/off

control/monitoring Short distance, few

feet maximum Single device

control/monitoring

Digital Input Example: Reading the status ofbuttons or switches


8/57


9/57

Digital Output : LED display


10/57

Digital Output : Relay Control


11/57

Analog I/O : Voltage-based controland monitoring

Simple interface Low cost for low-

resolutions High speed Low programming

overhead

High cost for higherresolutions

Not all uC haveanalog I/O built-in

Complicates thecircuit design when

external ADC orDAC are needed Short distance, few

feet maximum


12/57

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


13/57

Analog Interface


14/57

Parallel Bus

High speed High throughput:

Several bits aretransmitted on oneclock transition

Low cost

Large number ofmicrocontroller pins

needed forimplementing theparallel 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 )


15/57


16/57


17/57

Serial Buses : I 2 C (Inter IntegratedCircuit bus)

2-wire interface with one master and multipleslaves ( multi-master configurations possible)

Originated by Philips Semiconductor in theearly 80s to connect a microcontroller toperipheral devices in TV sets

Signals: DATA (SDA), CLOCK (SCL) and

Ground. SDA is always bi-directional; SCL isbi-directional only in multi-master mode Maximum allowable capacitance on the lines

is 400 pF. Typical device capacitance is 10 pF


18/57

Serial Buses : I 2 C (Inter IntegratedCircuit bus)

To start the communications, the bus master(typically a microcontroller) places theaddress of the device with which it intends tocommunicate (the slave) on the bus

All slave devices monitor the bus to determineif the master device is sending their address

Only the device with the correct addresscommunicates with the master By definition, I 2C is 5V


19/57

Multiple slave

devices - with only 3wires Low-cost to

implement Implemented in

hardware orsoftware

Ease to implement,many examples

Supports multi-master configuration

Short distance

Slow speed: 100KHz although 400KHz and 1 MHzslave device exist

These can notcoexist with slowerdevices

Limited deviceaddresses


20/57


21/57

Start & Stop

An I2

C master prepares to communicate with aslave device first by generating a Startcondition on the bus

Start condition is defined as SDA signal goinglow while SCL signal is high

Stop condition is defined as SDA going highwhile SCL is high


22/57

Data Validity

Data can change while the clock is low Data should remain stable while the clock is

going high


23/57

Acknowledge(ACK)


24/57

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


25/57

where Device Address is defined as : P0, P1,P2 indicate the page number ( 2Kbit pages ).A0, A1, A2 indicate the device number on thebus


26/57

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


27/57


28/57

SPI ( Serial Peripheral Interface )

SPI bus is a master/slave interface. Whenevertwo devices communicate, one is referred toas 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


29/57

Multiple slavedevices - with onlyfew wires

Low-cost HW/SW Implement Ease to implement,

many examples

Can be high speed( e.g. 4MHz orhigher ifimplemented inhardware )

Short distance Data and clock lines

can be shared buteach device requiresa separate ChipSelect signal, limitingthe number of

devices in limited I/Osystems


30/57

SPI bus specifies fourlogic signals

SCLK Serial Clock

(output from master) MOSI Master Output,Slave Input (output frommaster)

MISO Master Input,Slave Output (outputfrom slave)

SS Slave Select(active low; output frommaster

Sometimes, the followingnaming convention isused:

SCLK Serial Clock(output from master) SDI Serial Data In SDO Serial Data Out CS Chip Select

(active low; output frommaster)


31/57

SPI Bus with multiple slaves

M lti di C d ( MMC ) I t f


32/57

Multimedia Card ( MMC ) Interfaceusing SPI


33/57


34/57

Multiple slave devices-with only 2 wires Low-cost Implemented in HW/SW

Ease to implement,many examples Relatively long

distance. Theoretically

300 meters but this islimited in practice dueto noise and cablecapacitance

Slow speed 1-wire slave devices

typically has to comefrom one source:DallasSemiconductor

RS232


35/57

Asynchronous communications

Popular interface with many examples Many compatible legacy devices Relatively long distance, 50 feet maximum for

low baud rates although longer distanceswork in practice, with low baud rates and errorcorrection

Immune to noise due to +/-5 Volts or highervoltage levels for logic 0 and 1

Implemented in hardware or software Ease to implement, many examples


36/57

More suitable for system to systemcommunications, not so much for chip to chipor chip to sensor

Low speed for long distance, 115200 baudcan be achieved with small microcontrollersusing short distances

Requires transceiver chips which add tosystem cost ( TTL/CMOS level RS232 can beused without transceiver chips ).

Single master/single slave


37/57

RS485


38/57

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 forsystem to system

communications, not somuch for chip to chip orchip to sensor

Requires transceiverchips and twisted paircable with terminatingresistors which add tosystem cost


39/57

RS485 Network Topology: Any station cancommunicate with any other station, but not atthe same time


40/57

UART : Universal asynchronous


41/57

UART : Universal asynchronousreceiver/transmitter

UARTs are commonly used in conjunctionwith other communication standards such asEIA RS-232.

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

UARTs are now commonly included inmicrocontrollers. A dual UART or DUARTcombines two UARTs into a single chip.


42/57

As of now, UARTs are commonly used withRS-232 for embedded systemscommunications.

It is useful to communicate betweenmicrocontrollers and also with PCs.

Many chips provide UART functionality insilicon, and low-cost chips exist to convertlogic level signals (such as TTL voltages) toRS-232 level signals (for example, MaximMAX232)


43/57

h


44/57

Ethernet

Very high speed (10Mbit to 100 Mbit/s )

Very long distance,hundreds of feet canbe achieved, morewith hubs andswitches

Immune to noise Widely used in

industrial automationdue to noise immunity

Cost More suitable for system

to systemcommunications, not somuch for chip tochip/sensor

Requires Ethernet chipset,transformer, jack andspecial cabling that add tosystem cost.

Complicated to implement High code footprint


45/57

P i


46/57

Programming

Hardware interface Software emulation : Bit banging

Bit b i


47/57

Bit banging

Bit-banging is a technique in embeddedsystems for example to use serialcommunications without the use of dedicated

hardware such as a UART or shift register,instead using software to emulate theirbehavior

A software routine handles the UART transmitfunction by alternating a pin on themicrocontroller by given time intervals

Bit b gi g


48/57

Bit banging

A receiver function is implemented bysampling a pin on the microcontroller by agiven 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 extremeextension of the same idea


49/57

Bit banging


50/57

Bit banging

Another problem is that the microcontroller isbusy most of the time looking at samples orsending a sample to the pin, instead of

performing other tasks Yet another potential problem is that the

signal produced normally has more jitter orglitches, especially when the microcontrollermoves on to perform other tasks.

However, if the bit-banging software ishardware interrupt-driven by the signal, this

may be of minor importance

More Buses


51/57

More Buses

USB RS232 UART RS-422

CAN Diff between TWI and I 2C FIREWIRE


52/57

CAN


53/57

CAN ...

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

Although initially created for automotive

purposes (as a vehicle bus), nowadays it isused 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 aHamming bit length of 6 (so up to 5 bits in arow corrupted will be detected by any node on

CAN


54/57

CAN ...

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

USB


55/57

USB

Universal Serial Bus (USB) is a serial busstandard to interface devices It was originally designed for computers, but

its popularity has prompted it to also becomecommonplace on video game consoles,PDAs, portable DVD and media players,cellphones; and even devices such astelevisions, home stereo equipment, carstereos and portable memory devices

The radio spectrum based USBimplementation is known as Wireless USB

USB : Overview


56/57

USB : Overview

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

The intention was to let go of all older serial

and parallel ports on personal computerssince these were not properly standardizedand required a multitude of device drivers tobe developed and maintained

A USB system has an asymmetric design,consisting of a host controller and multipledaisy-chained devices

USB : Overview


57/57

USB : Overview

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

No more than 127 devices, including the bus

devices, may be connected to a single hostcontroller Modern computers often have several host

controllers, allowing a very large number ofUSB devices to be connected

USB cables do not need to be terminated

Interfacing mc

Documents