Tutorial on AT Commands for GSM/GPRS with microcontrollers

[SevenZero]

AT commands are used to control MODEMs. AT is the abbreviation for Attention. These commands come from Hayes commands that were used by the Hayes smart modems. The Hayes commands started with AT to indicate the attention from the MODEM. The dial up and wireless MODEMs (devices that involve machine to machine communication) need AT commands to interact with a computer. These include the Hayes command set as a subset, along with other extended AT commands.

If you use a GSM/GPRS module such as SIM900, you will need to use these commands.

AT commands with a GSM/GPRS MODEM or mobile phone can be used to access following information and services:
Information and configuration pertaining to mobile device or MODEM and SIM card.

1. SMS services
2. MMS services
3. Fax services
4. Data and Voice link over mobile network

The Hayes subset commands are called the basic commands and the commands specific to a GSM network are called extended AT commands.

Types of AT Commands
There are four types of AT commands as below.

AT.jpg (16.28 KiB) Viewed 17710 times

1. Test commands - used to check whether a command is supported or not by the MODEM.
   SYNTAX: AT<command name>=?
   For example: ATD=?
2. Read command - used to get mobile phone or MODEM settings for an operation.
   SYNTAX: AT<command name>?
   For example: AT+CBC?
3. Set commands - used to modify mobile phone or MODEM settings for an operation.
   SYNTAX: AT<command name>=value1, value2, …, valueN
   Some values in set commands can be optional.
   For example: AT+CSCA=”+9876543210”, 120
4. Execution commands - used to carry out an operation.
   SYNTAX: AT<command name>=parameter1, parameter2, …, parameterN
   The read commands are not available to get value of last parameter assigned in execution commands because parameters of execution commands are not stored.
   For example: AT+CMSS=1,”+ 9876543210”, 120

Commonly used AT commands in detail
Explanation of commonly used AT commands:

1. AT - This command is used to check communication between the module and the computer.
   For example,
   AT
   OK
   The command returns a result code OK if the computer (serial port) and module are connected properly. If any of module or SIM is not working, it would return a result code ERROR.
2. +CMGF - This command is used to set the SMS mode. Either text or PDU mode can be selected by assigning 1 or 0 in the command.
   SYNTAX: AT+CMGF=<mode>
   0: for PDU mode
   1: for text mode
   
   The text mode of SMS is easier to operate but it allows limited features of SMS. The PDU (protocol data unit) allows more access to SMS services but the operator requires bit level knowledge of TPDUs. The headers and body of SMS are accessed in hex format in PDU mode so it allows availing more features.
   
   For example,
   AT+CMGF=1
   OK
3. +CMGW - This command is used to store message in the SIM.
   SYNTAX: AT+CMGW=” Phone number”> Message to be stored Ctrl+z
   As one types AT+CMGW and phone number, ‘>’ sign appears on next line where one can type the message. Multiple line messages can be typed in this case. This is why the message is terminated by providing a ‘Ctrl+z’ combination. As Ctrl+z is pressed, the following information response is displayed on the screen.
   
   +CMGW: Number on which message has been stored
4. +CMGS - This command is used to send a SMS message to a phone number.
   SYNTAX: AT+CMGS= serial number of message to be send.
   
   As the command AT+CMGS and serial number of message are entered, SMS is sent to the particular SIM.
   For example,
   AT+CMGS=1
   OK
5. ATD - This command is used to dial or call a number.
   SYNTAX: ATD<Phone number>(Enter)
   For example,
   ATD123456789
6. ATA - This command is used to answer a call. An incoming call is indicated by a message ‘RING’ which is repeated for every ring of the call. When the call ends ‘NO CARRIER’ is displayed on the screen.
   SYNTAX: ATA(Enter)
   
   As ATA followed by enter key is pressed, incoming call is answered.
   For example,
   RING
   RING
   ATA
7. ATH - This command is used to disconnect remote user link with the GSM module.
   SYNTAX: ATH (Enter)

List of AT commands
The AT commands for both, GSM module and the mobile phone, are listed below. Some of these commands may not be supported by all the GSM modules available. Also there might be some commands which won’t be supported by some mobile handsets.

Testing
Command	Description
AT	Checking communication between the module and computer

Call control
Command	Description
ATA	Answer command
ATD	Dial command
ATH	Hang up call
ATL	Monitor speaker loudness
ATM	Monitor speaker mode
ATO	Go on-line
ATP	Set pulse dial as default
ATT	Set tone dial as default
AT+CSTA	Select type of address
AT+CRC	Cellular result codes

Data card Control
Command	Description
ATI	Identification
ATS	Select an S-register
ATZ	Recall stored profile
AT&F	Restore factory settings
AT&V	View active configuration
AT&W	Store parameters in given profile
AT&Y	Select Set as power up option
AT+CLCK	Facility lock command
AT+COLP	Connected line identification presentation
AT+GCAP	Request complete capabilities list
AT+GMI	Request manufacturer identification
AT+GMM	Request model identification
AT+GMR	Request revision identification
AT+GSN	Request product serial number identification (IMEI)

Phone control
Command	Description
AT+CBC	Battery charge
AT+CGMI	Request manufacturer identification
AT+CGMM	Request model identification
AT+CGMR	Request revision identification
AT+CGSN	Request product serial number identification
AT+CMEE	Report mobile equipment error
AT+CPAS	Phone activity status
AT+CPBF	Find phone book entries
AT+CPBR	Read phone book entry
AT+CPBS	Select phone book memory storage
AT+CPBW	Write phone book entry
AT+CSCS	Select TE character set
AT+CSQ	Signal quality

Computer data interface
Command	Description
ATE	Command Echo
ATQ	Result code suppression
ATV	Define response format
ATX	Response range selection
AT&C	Define DCD usage
AT&D	Define DTR usage
AT&K	Select flow control
AT&Q	Define communications mode option
AT&S	Define DSR option
AT+ICF	DTE-DCE character framing
AT+IFC	DTE-DCE Local flow control
AT+IPR	Fixed DTE rate

Service
Command	Description
AT+CLIP	Calling line identification presentation
AT+CR	Service reporting control
AT+DR	Data compression reporting
AT+ILRR	DTE-DCE local rate reporting

Network Communication parameter
Command	Description
ATB	Communications standard option
AT+CBST	Select bearer service type
AT+CEER	Extended error report
AT+CRLP	Radio link protocol
AT+DS	Data compression

Miscellaneous
Command	Description
A/	Re-execute command line
AT?	Command help
AT*C	Start SMS interpreter
AT*T	Enter SMS block mode protocol
AT*V	Activate V.25bis mode
AT*NOKIATEST	Test command
AT+CESP	Enter SMS block mode protocol

SMS Text mode
Command	Description
AT+CSMS	Select message service
AT+CPMS	Preferred message storage
AT+CMGF	Message format
AT+CSCA	Service centre address
AT+CSMP	Set text mode parameters
AT+CSDH	Show text mode parameters
AT+CSCB	Select cell broadcast message types
AT+CSAS	Save settings
AT+CRES	Restore settings
AT+CNMI	New message indications to TE
AT+CMGL	List messages
AT+CMGR	Read message
AT+CMGS	Send message
AT+CMSS	Send message from storage
AT+CMGW	Write message to memory
AT+CMGD	Delete message

SMS PDU mode
Command	Description
AT+CMGL	List Messages
AT+CMGR	Read message
AT+CMGS	Send message
AT+CMGW	Write message to memory

[SevenZero]

Here are some objectives that are fulfilled using AT Commands:

1. Test the simple AT Command.
2. Find out the IMEI number of the GSM Modem.
3. Connect a call to a GSM mobile number (Dial a number).
4. Send a text message to a mobile number.

AT Commands used to perform the above operations have been given below along with their output. (For complete list of AT Commands supported by GSM Modem, refer the list in tutorial on AT Commands.

1. Test the simple AT Command
   AT (Enter)
   OK
2. Find out the IMEI number of the GSM modem
   AT+GSN (Enter)
   xxxxxxxxxxxxxxx(15- digit unique IMEI number)
   OK
3. Dial a number
   ATDXXXXXXXXXX; (Enter) (10-digit mobile number)
   OK
4. Send a text message.
   AT+CMGS= “XXXXXXXXXX” (Enter) (10-digit mobile number)
   >Hello ^z (Enter message after ‘>’ and use Ctrl+z to terminate the message)

Programming Steps:

1. Store all AT commands into strings
2. Set the baud rate of microcontroller to 9600bps
3. Enable serial port. (Refer USART with PIC)
4. Enable the global and peripheral interrupt bits of the INTCON register
5. Configure Port D as input port
6. Use if condition to detect the pressed switch
7. As switch is pressed, process the corresponding AT command and transmit via USART
8. Reception interrupt method is used to store the GSM output data into an array
9. Display the stored value on the LCD. (Refer displaying text on LCD using PIC)

[SevenZero]

Sample circuit diagram on connecting a GPRS module such as the ones you can find from TRONIC.LK.

GSM_CD.gif (21.89 KiB) Viewed 17694 times

Here is a sample code is C language:

Code: Select all

/*
This code takes four choices as four inputs

Choice 1 : Test the simple AT Command
Choice 2 : Find out the IMEI number of the GSM Modem
Choice 3 : Connect a call to a GSM mobile number
Choice 4 : Send a text message to a mobile number
*/

#define FREQ 12000000
#define baud 9600
#define spbrg_value (((FREQ/64)/baud)-1)
#define rs LATA.F0
#define rw LATA.F1
#define en LATA.F2
#define lcdport LATB

void tx_data(unsigned char);
unsigned char rx_data();
void lcd_ini();
void lcdcmd(unsigned char);
void lcddata(unsigned char);
void gsm_cmd(unsigned char *);
void output(void);

unsigned char value = 0;
int i=0,j,k,temp,flag,choice;
unsigned char *starting_text = "Enter choice=";
unsigned char *dial_text = "Dialing...";
unsigned char *at_cmd = "AT";
unsigned char *imei_cmd = "AT+GSN";
unsigned char *call_cmd = "ATD9xxxxxxxxx;"; // Provide a 10-Digit Mobile Number
unsigned char *sms_format = "AT+CMGF=1";
unsigned char *sms_write = "AT+CMGS=\"xxxxxxxxxx\""; // 10-Digit Mobile Number
unsigned char *sms = "Hello";
unsigned char *sms_report = "SMS Sent...";
unsigned char sms_terminate = 0x1A;
unsigned char enter = 0x0D;
unsigned char *data;

void main()
{
	TRISB=0; // Set Port B as output port
	LATB=0;
	TRISA=0;
	LATA=0;
	TRISD=0xFF;
	LATD=0;
	SPBRG=spbrg_value; // Fill SPBRG register to set the baud rate
	RCSTA.SPEN=1; // To activate serial port (Tx and Rx pins)
	TXSTA.TXEN=1; // Activate Transmissiom
	RCSTA.CREN=1; // Activate Reception
	PIE1.RCIE=1; // Enable Reception interrupt
	INTCON.GIE=1; // Enable Global interrupt
	INTCON.PEIE=1; // Enable Peripheral interrupt

	lcd_ini();
	while(1)
	{
		k = 0;
		lcdcmd(0x80);
		while(starting_text[k]!='\0')
		{
			lcddata(starting_text[k]);
			k++;
		}

		//Check inputs

		//Choice 1
		if(PORTD.F0)
		{
			gsm_cmd(at_cmd);
			output();
			Delay_ms(1000);
		}


		//Choice 2
		if(PORTD.F1)
		{
			gsm_cmd(imei_cmd);
			output();
			Delay_ms(1000);
		}

		//Choice 3
		if(PORTD.F2)
		{
			gsm_cmd(call_cmd);
			output();
			Delay_ms(1000);
		}

		//Choice 4
		if(PORTD.F3)
		{
			gsm_cmd(sms_format);
			output();
			Delay_ms(1000);

			gsm_cmd(sms_write);
			output();
			Delay_ms(1000);

			gsm_cmd(sms);
			output();
			tx_data(0x1A);
			Delay_ms(1000);
		}

	}

}

void gsm_cmd(unsigned char *string)
{
	i=0;j=0;
	while(string[i]!='\0')
	{
		temp=0;
		if(string[i]==0x5C) // Not to send '\' cahracter
		i++;
		tx_data(string[i]); // Send by serial communication
		i++;
		while(temp!=1);
	}
	temp=0;
	tx_data(enter); // Send ASCII code for 'Enter' key
	while(temp!=1);
}

void output(void) // To print data on LCD
{
	lcdcmd(0x01);
	i=-1;flag=0;
	while(i<j)
	{
		if(flag>1)
		{
			flag=0;
			Delay_ms(500);
			lcdcmd(0x01);
			lcdcmd(0x80);
		}
		if(data[i]==0x0A) // This condition is to avoid double Enter
		// during execution of a command
		{
			flag++;
			lcdcmd(0xc0);
		}
		if(data[i]=='>'||data[i]=='"') // Not to print this character
		{
			i++;
			lcdcmd(0xc0);
		}
		if(data[i]!=0x0D&&data[i]!=0x0A&&data[i]!=0x1A) // Condition to print the data
		// except 'Enter','New line' and 'Submit'
		{
			lcddata(data[i]);
			i++;
		}
		else
			i++;
		Delay_ms(300);
	}
	lcdcmd(0x01);
}

void tx_data(unsigned char serial_data) // Transmit data function
{
	TXREG = serial_data;
	while(PIR1.TXIF==0);
}


void interrupt()
{
	data[j]=RCREG; // Store the data into array when Reception interrupt occurs
	value=RCREG;
	j++;
	temp=1;
}

void lcd_ini()
{
	lcdcmd(0x38); // Configure the LCD in 8-bit mode, 2 line and 5x7 font
	lcdcmd(0x0C); // Display On and Cursor Off
	lcdcmd(0x01); // Clear display screen
	lcdcmd(0x06); // Increment cursor
	lcdcmd(0x80); // Set cursor position to 1st line, 1st column
}

void lcdcmd(unsigned char cmdout)
{
	lcdport=cmdout; //Send command to lcdport=PORTB
	rs=0;
	rw=0;
	en=1;
	Delay_ms(10);
	en=0;
}

void lcddata(unsigned char dataout)
{
	lcdport=dataout; //Send data to lcdport=PORTB
	rs=1;
	rw=0;
	en=1;
	Delay_ms(10);
	en=0;
}

[nipunalanka]

this cct can use phone,not chip

[nipunalanka]

no i wont connect to nokia 1110 or 1100 phone friend

[Neo]

Nipuna, it is not possible to use AT commands with Nokia 1100 as there is no modem in there. But Nokia F-BUS protocol is there. But you need to use a different protocol to use that. Search for Nokia F-Bus.

Here are the connecters for f-bus.

Nokia 1100_1.jpg (7.53 KiB) Viewed 17677 times

Nokia 1100_2.jpg (5.58 KiB) Viewed 17677 times

Nokia 1100_3.gif (2.58 KiB) Viewed 17677 times

PIN	DESCRIPTION
1	DAI CLK
2	F-Bus Rx
3	F-Bus Tx
4	Gnd
5	M-Bus
6	Vpp
7	Gnd
8	BSI
9	Vcc

Here is the link for Nokia 1100 schematics.
http://www.cpkb.org/wiki/Nokia_1100_110 ... c_download

This link has some good information about F-BUS.
http://www.embedtronics.com/nokia/fbus.html

[maria752]

Have you any idea about HTC mobile circuit.... i have found Mic problem but it is hard to task to compile the solution. according to your upper solution this become very beneficiary for me....