AT Commands

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Published On:

Aug 13, 2014

Last Updated:

Aug 13, 2014

AT commands are an text-based control protocol primarily used for configuring and controlling cellular modems from host microcontrollers or manual typing. AT is an abbreviation of “ATtention”. AT commands are usually sent across a UART serial connection. Because they are text-based, it is also easy to control a modem connected to a terminal program (such as NinjaTerm) via serial.

The nRF91 series of SiPs (system-in-packages) from Nordic Semiconductor include modems which provide the AT command protocol for configuration and control.

Protocol

AT commands are text-based and limited to the ASCII character set. Each command and response is terminated with either a carriage return (CR) and line feed (LF) or just a carriage return. This is modem dependent and sometimes configurable. Although rare, some commands “bend” the rules and allow line termination inside the command. For example, the Nordic Semiconductor nRF91 modems allow the command AT%CMNG to send a multiline certificate in .pem format as part of the command. For example (this is sending the DigiCertGlobalRootG2 certificate):

AT%CMNG=0,16842753,0,"-----BEGIN CERTIFICATE-----
MIIDjjCCAnagAwIBAgIQAzrx5qcRqaC7KGSxHQn65TANBgkqhkiG9w0BAQsFADBh
MQswCQYDVQQGEwJVUzEVMBMGA1UEChMMRGlnaUNlcnQgSW5jMRkwFwYDVQQLExB3
d3cuZGlnaWNlcnQuY29tMSAwHgYDVQQDExdEaWdpQ2VydCBHbG9iYWwgUm9vdCBH
MjAeFw0xMzA4MDExMjAwMDBaFw0zODAxMTUxMjAwMDBaMGExCzAJBgNVBAYTAlVT
MRUwEwYDVQQKEwxEaWdpQ2VydCBJbmMxGTAXBgNVBAsTEHd3dy5kaWdpY2VydC5j
b20xIDAeBgNVBAMTF0RpZ2lDZXJ0IEdsb2JhbCBSb290IEcyMIIBIjANBgkqhkiG
9w0BAQEFAAOCAQ8AMIIBCgKCAQEAuzfNNNx7a8myaJCtSnX/RrohCgiN9RlUyfuI
2/Ou8jqJkTx65qsGGmvPrC3oXgkkRLpimn7Wo6h+4FR1IAWsULecYxpsMNzaHxmx
1x7e/dfgy5SDN67sH0NO3Xss0r0upS/kqbitOtSZpLYl6ZtrAGCSYP9PIUkY92eQ
q2EGnI/yuum06ZIya7XzV+hdG82MHauVBJVJ8zUtluNJbd134/tJS7SsVQepj5Wz
tCO7TG1F8PapspUwtP1MVYwnSlcUfIKdzXOS0xZKBgyMUNGPHgm+F6HmIcr9g+UQ
vIOlCsRnKPZzFBQ9RnbDhxSJITRNrw9FDKZJobq7nMWxM4MphQIDAQABo0IwQDAP
BgNVHRMBAf8EBTADAQH/MA4GA1UdDwEB/wQEAwIBhjAdBgNVHQ4EFgQUTiJUIBiV
5uNu5g/6+rkS7QYXjzkwDQYJKoZIhvcNAQELBQADggEBAGBnKJRvDkhj6zHd6mcY
1Yl9PMWLSn/pvtsrF9+wX3N3KjITOYFnQoQj8kVnNeyIv/iPsGEMNKSuIEyExtv4
NeF22d+mQrvHRAiGfzZ0JFrabA0UWTW98kndth/Jsw1HKj2ZL7tcu7XUIOGZX1NG
Fdtom/DzMNU+MeKNhJ7jitralj41E6Vf8PlwUHBHQRFXGU7Aj64GxJUTFy8bJZ91
8rGOmaFvE7FBcf6IKshPECBV1/MUReXgRPTqh5Uykw7+U0b6LJ3/iyK5S9kJRaTe
pLiaWN0bfVKfjllDiIGknibVb63dDcY3fe0Dkhvld1927jyNxF1WW6LZZm6zNTfl
MrY=
-----END CERTIFICATE-----"

The simplest command is just simply AT which the modem will respond to with OK.

Proprietary commands are usually prefixed with the % character, e.g. Nordic Semiconductor’s nRF91 SiP modems have a AT%CMNG command to manage credentials (certificates and keys).

Commands

AT

This command is used to test if the modem is responding. It should respond with OK. The command is just AT followed by a carriage return and line feed.

AT+CGMR

This command is used to get the modem firmware version.

AT+CMGF

This command sets up the SMS mode of the modem.

AT+CMGW

This command is used to store a message in the memory of the SIM card.

Echo

Echoing is a common feature which is enabled when talking with devices using the AT command protocol. An important thing to note is the device will echo letters as they are typed into the terminal, not when you press enter. This may have implications when trying to use echo to match a sent command with a response.

AT Result Codes

It can be hard to determine the exact cause of an error because the modem usually just responds with ERROR with no further information.

The following table lists the most commonly used AT result codes:

Result Code (Numerical)	Result Code (Textual)
0	OK
1	CONNECT
2	RING
3	NO CARRIER
4	ERROR
5	CONNECT 1200
6	NO DIALTONE
7	BUSY
8	NO ANSWER