In this article:

This guide provides comprehensive instructions on how to set up, configure, and communicate with the AT Command Modem. It covers hardware connection, serial communication settings, the AT command protocol, and common troubleshooting steps.

Initial Setup

Before you can send commands, you need to connect the hardware and configure your serial terminal correctly.

Hardware Connection

Connect the following pins from your host system to the modem:

• LEUART Tx/Rx
• GND
• GPIO7 (Optional: This pin outputs high to indicate a job is running)

For detailed electrical characteristics and integration guidance, refer to the Myriota Modem Application Note.

Serial Communication Configuration

Use the following settings in your serial terminal application (e.g., PuTTY, minicom, Tera Term):

• Baudrate: 9600
• Data bits: 8
• Stop bits: 1
• Parity: None
• Flow control: None

Understanding the AT Command Protocol

The modem uses a standard AT command set for communication. Here are the fundamental rules.

Command Structure

• Prefix: All commands must begin with AT.
• Case Sensitivity: Commands are case-sensitive and must be in UPPERCASE. For example, AT+MSGQ=? is valid, but at+msgq=? is not.
• Terminators: Commands are terminated by a carriage return (<CR>), a line feed (<LF>), or a space. It's best practice to use <CR><LF> at the end of each command.
• Length: The maximum command length, including the AT prefix and terminators, is 80 characters.

Power-Up Sequence 

On power-up, the modem will send status messages to the host. You must wait for the +STATE=READY message before you can schedule a message for transmission.

1. +STATE=INITIALIZING: The system is starting up.
2. +STATE=GNSS_ACQ: The modem is attempting to acquire a GNSS fix.
3. +STATE=READY: A GNSS fix has been acquired, and the modem is ready for all operations.

Command Types & Responses

There are two main types of commands:

• Query Commands (AT+<CMD>=?): Used to read data or status from the modem.
  • Success Response: OK+<CMD>=<VALUE>
  • Failure Response: FAIL+<CMD>=<VALUE>
• Control Commands (AT+<CMD>=<PARAMETER>): Used to perform an action or change a setting.
  • Success Response: OK+<CMD> or OK+<CMD>=<PARAMETER>
  • Failure Response: FAIL+<CMD> or FAIL+<CMD>=<PARAMETER>

A simple AT command is used to check communication and will return OK if successful.

Commands with Examples

Here are some of the most frequently used query and control commands. For a complete list and advanced details, please refer to the at_modem README on GitHub.

Query Commands

• Check Communication
  • Usage: AT
  • Example Response: OK
• Message Queue
  • Usage: AT+MSGQ=?
  • Example Response: OK+MSGQ=10

Note: Returns the number of free slots in the message queue.

• System State
  • Usage: AT+STATE=?
  • Example Response: OK+STATE=READY

Note: States are: INITIALIZING, GNSS_ACQ or READY.

• SDK Version
  • Usage: AT+VSDK=?
  • Example Response: OK+VSDK=1.2.3
• Module ID
  • Usage: AT+MID=?
  • Example Response: OK+MID=0012345678 M1-24
• Registration Code
  • Usage: AT+REGCODE=?
  • Example Response: OK+REGCODE=a1b2c3d4e5f6g7h8i9j0k1l2m
• Time
  • Usage: AT+TIME=?
  • Example Response: OK+TIME=1673395800
• Location
  • Usage: AT+LOCATION=?
  • Example Response: OK+LOCATION=-349205499,1386086737

Note: The latitude and longitude are scaled by 1e7.

• Get suspend mode
  • Usage: AT+SUSPEND=?
  • Example Response: OK+SUSPEND=0

Control Commands

• Save Message Queue
  • Usage: AT+SAVEMSG
  • Example Response: OK+SAVEMSG
• Start RF TX Test
  • Usage: AT+TXSTART=<FREQUENCY>,<TX_TYPE>,<BURST_MODE>,<TIMEOUT>
  • Example Usage: AT+TXSTART=161450000,0,1,60
  • Example Response: OK+TXSTART=161450000,0,1,60
• Stop RF TX Test
  • Usage: AT+TXSTOP
  • Example Response: OK+TXSTOP
• Start GNSS Fix
  • Usage: AT+GNSSFIX
  • Example Response: OK+GNSSFIX

Note: Returns OK+GNSSFIX immediately, then returns a second OK+GNSSFIX on a successful fix, or FAIL+GNSSFIX after a 90s timeout.

• RSSI Test
  • Usage: AT+RSSI=<FREQUENCY>
  • Example Usage: AT+RSSI=360000000
  • Example Response: OK+RSSI=-95
• Schedule a Message
  • Usage: AT+SMSG=<PARAMETER>
  • Example Usage: AT+SMSG=0102030405060708091011121314151617181920
  • Example Response: OK+SMSG=0102030405060708091011121314151617181920

Note: The message to be sent, represented as a hex string of even length.

• Suspend Mode
  • Usage: AT+SUSPEND=<PARAMETER>
  • Example Usage: AT+SUSPEND=1
  • Example Response: OK+SUSPEND=1
• Set Time
  • Usage: AT+TIME=<PARAMETER>
  • Parameter: The current time as a Unix epoch timestamp.
  • Example Usage: AT+TIME=1673395800
  • Example Response: OK+TIME=1673395800
• Set Location
  • Usage: AT+LOCATION=<PARAMETER>
  • Parameter: The latitude and longitude, each scaled by 1e7 and separated by a comma.
  • Example Usage: AT+LOCATION=-349205499,1386086737
  • Example Response: OK+LOCATION=-349205499,1386086737

Troubleshooting and Error Codes

If a command fails, the modem will return an error code. The timeout for any response is 2 seconds, after which the host can retry the command.

Common Mistakes

• Incorrect Case: Sending at+state=? will fail because commands must be uppercase. This results in an ERROR=UNKNOWN_CONTROL_CMD.
• Invalid SMSG Parameter: The message payload for AT+SMSG must be a valid hexadecimal string with an even number of characters. AT+SMSG=...2 (odd length) or AT+SMSG=...G (invalid hex character) will return ERROR=INVALID_PARAMETER.
• Invalid LOCATION Format: The location parameter must contain two numbers separated by a comma. AT+LOCATION=-349205499.1386086737 (using a period) will fail.

Error Code Table