Matlab Serial Port

Matlab Serial PortPractical notes on Matlab serial port programming

by

Roberto MeattiniPhD student in Robotics and Automatic Control @ University of Bologna

roberto [dot] meattini2 [at] unibo [dot] it

Matlab interfaces with serial ports through the SERIAL PORT OBJECT

but first…

by Roberto Meattini - PhD student in Robotics and Automatic Control @ University of Bologna

2

Serial port overview

• Serial communication• most used low-level protocol to connect devices

• send/receive bytes in a serial way (1 bit at a time)

• Serial data format• start bit

• 5 to 8 data bits

• parity bit

• 1 to 2 stop bits


3

start data data data data data data data data parity stop stop

Decription format: n. data bits – parity type – n. stop bitsExample: 8-N-2

Serial port overview (I)

• Synchronous vs. asynchronous• sync: all transmitted bits are synchronized with a common clock

(start/stop bits are not necessary)

• async: every device has its own internal clock and data is transmitted at arbitrary time(start/stop bits are necessary)

• Baud rate• n. bits transferred per second

• OS serial port• in the following we will refer to Windows OS COM1, COM2, …


4

The Matlab serial port session

• Example:

s=serial(‘COM1’) create the serial port objectset(s,’BaudRate’,4800); configurate propertiesfopen(s); connection to the devicefprintf(s,’*IDN?’) read/write dataout=fscanf(s); read/write datafclose(s) disconnectiondelete(s) delete port bojectclear s clean


5

The serial port session (I)

• set(s)

• shows the list of all properties

• get(s)

• shows the list of all properties with respective values

• s.BaudRate

• the way to show a single property value

• set(s,’BaudRate’,4800)

• the way to set a property value


6

Create a Serial Port Object

• s = serial(‘<port_name>’);

• the port name depends on the OS (Windows COM1,COM2, ...)

• instrhwinfo(‘serial’)

• it is a Control Instrument Toolbox command that return a list of available serial port

• Once the port object is created, typings ,

we obtain an object display summary withserial port obj name, communication settings, communication state, read/write operations state


7

Communication settings

• BaudRate bits per second

• DataBits specifies n. bits to be transmitted

• Parity specifies the parity checking type

• Terminator specifies the termination character


8

Writing and reading data

• Synchronous communication• transmission is synchronized using start and stop bits and/or a common clock

between devices• in this case, the read/write operations block the access to the Matlab command line

• Asynchronous communication• read/write operations are not blocking

• it is possible to read and write simultaneously

• callback functions can be used


9

Writing data

• Writing functions• fprintf writes text on the device• fwrite writes binary data on the device• stopasync stops write asynchronous operations

• Writing properties• BytesOutput n. bytes present in the output buffer• OutputBufferSize output buffer’s dimension (in bytes)• Timeoutmax waiting time to complete a write operation• TransferStatus indicates if an asynchronous write operation is in

progress• ValuesSent n. values written on the device


10

Writing data (I)

• Synchronous vs. asynchronous• fprintf and fwrite are synchronous by default

• to write in asynchronous mode we have to explicit it as an argument

• example:

fprintf(s, ’<…text…>’, ’async’)

• in this case it is possible to write and read simultaneously, and to use callback functions


11

Reading data

• Reading functions• fgetl reads a text line and discards the terminator• fgets reads a text line and includes the terminator• fread reads binary data• fscanf reads data from the device as text• readasync reads data asynchronously• stopasync stops read asynchronous operations

• Reading properties• BytesAvailable n. bytes available in the input buffer• InputBufferSize input buffer’s dimension (in bytes)• ReadAsyncMode specifies if an asynchronous read operation is “continuous” or

“manual”• Timeoutmax waiting time to complete a read operation• TransferStatus indicates if an asynchronous read operation is in progress• ValuesReceived n. values read from the device


12

Reading data (I)

• Synchronous vs. asynchronous• if ReadAsyncMode is set to

• ‘continuous’ (default value) the Serial Port Object continuously checks if there are available data from the device and, if it is present, stores it asynchronously into the input buffer. Then it is possible to use a synchronous function to transfer data to Matlab(fget/fread/fscan)

• ‘manual’ the Serial Port Object does not check continuously the device … …

• remind: with asynchronous mode it is possible to write and read simultaneously, and to use callback functions


13

Reading data (II)

• fscanf operation• blocks access to the command line until:

• the terminator is read• timeout is expired• n. of specified values are read• input buffer is full

• fread operation• it does not care about the terminator because reads directly the number of specified

values (by default bytes). Example:s.BytesAvailable = 69;

out = fread(s, 69) reads directly 69 bytes and stores in the variable “out”


14

CallbackA function executed when an event occurs, specifying the name of the callback function as a

value of the associated callback property


15

Events and Callbacks

• Event: BREAK INTERRUPT• Associated properties: BreakInterruptFcn

• the serial port generate a break interrupt when the received data stay inactive for a period greater than a character transmission time

• Event: BYTES AVAILABLE• Associated properties: BytesAvailableFcn

BytesAvailableFcnCount

BytesAvailableFcnMode

• if BytesAvailableFcnMode is set as ‘byte’, the callback specified in BytesAvailableFcn is executed every time there are as many bytes as in BytesAvailableFcnCount

• if BytesAvailableFcnMode is set as ‘terminator’, the callback is executed when the terminator character is read

• N.b.: this event can be generated only during asynchronous read operations


16

Events and Callbacks (I)

• Event: ERROR• Associated properties: ErrorFcn

• an error event occurs when read/write operation are not complited in the time specified in Timeout

• only for asynchronous read/write

• Event: OUTPUT EMPTY• Associated properties: OutoutEmptyFcn

• only for asynchronous write operations

• Event: PIN STATUS• Associated properties: PinStatusFcn

• associated to the change of the values of some control pins … …• for both sync and async operations


17

Events and Callbacks (II)

• Event: TIMER• Associated properties: TimerFcn

TimerPeriod

• this event is generated when the time specified in TimerPeriod expires, starting from the serial obj connection time; callback specified in TimerFcn is executed

• based on the “system velocity” the TimePeriod value is lower bounded


18

Events and Callbacks (III)

• Event information• composed by Type and Data values specified in the callback function header

• Type event name• Data.AbsTime day-month-year hour:minute:second• for the TIMER event there is also: Data.Message <…stringa errore…>• more Data values for the event PIN STATUS event … …

• Executing callback functions• we need to set the value of the callback property as the name of file with the code of

the callback• example:

s.ByteAvailableFcnMode = ‘terminator’;

s.ByteAvailableFcn = @my_callback;

• to pass additional parameters, example: s.ByteAvailableFcn = {@my_callback,val1,val2};


19

Events and Callbacks (IV)

• Callback file, example:function my_callback(obj,event[…others possible parameters …])

…

…

…

end

• If an error occurs during the callback execution• the callback is automatically disabled

• a warning is shown


20

Save and loading

• myserial file

• s serial port obj

• out variable with read data

save myserial s out stores data and serial obj

load myserial recreates s and out in the workspace


21

REFERENCE:

http://it.mathworks.com/help/matlab/serial-port-devices.html


22

http://it.mathworks.com/help/matlab/serial-port-devices.html

Matlab Serial Port

Engineering