Cellular at modem integration

Cellular AT Modem Integration

Overview

Use this skill to drive a 2G/4G/NB-IoT/LTE-M cellular modem from an MCU over UART with the AT command set. The hard parts are parsing unsolicited result codes (URCs) that interleave with command responses on one serial line, choosing the right registration command per radio access technology, using the correct socket command set for the specific module family, and getting PWRKEY boot timing right before any AT works.

When To Use

Use this skill when:

• The user integrates Quectel (EC25, BG96, BC66), SIMCom (SIM7600, SIM800C, A7670), or u-blox (SARA-R4, SARA-R5) over UART.
• The issue involves registration (AT+CPIN?, AT+CSQ, AT+CREG?, AT+CGREG?, AT+CEREG?), APN/PDP (AT+CGDCONT, AT+CGATT, AT+CGACT), or sockets (AT+QIOPEN, AT+QISEND, AT+CIPSTART/AT+CIPOPEN, AT+USOCO).
• URCs such as RING, +CMTI, or +QIURC corrupt command parsing, or power saving (AT+CPSMS, AT+CEDRXS) breaks reachability.

Do not use this skill when the UART link itself is unproven; confirm the modem echoes AT with OK at the configured baud first.

First Questions

Ask for:

• Exact module and firmware revision (ATI, AT+CGMR), and radio technology: 2G, LTE Cat-1, Cat-4, Cat-M1, or NB-IoT.
• UART wiring: baud, flow control (RTS/CTS), and whether PWRKEY/reset lines are MCU-controlled.
• SIM type and the operator APN (some IoT SIMs need username/password or a specific <PDP_type>).
• Transport goal: raw TCP/UDP, or bridging to MQTT/TLS on top of the socket AT layer.
• Current failing command and its literal response or error code.

Integration Checklist

1. Bring up the modem. Assert PWRKEY for the module's specified pulse width, wait for boot (often signalled by an RDY URC), then confirm AT returns OK and disable echo with ATE0.

2. Unlock and check signal. AT+CPIN? must return +CPIN: READY; AT+CSQ RSSI must be non-99 (0-31 valid). Set AT+CFUN=1 if radio is off.

3. Confirm registration on the right command. Use AT+CREG? for 2G/CS registration, AT+CGREG? for GPRS/PS, and AT+CEREG? for LTE/Cat-M1/NB-IoT (EPS). <stat> must be 1 (home) or 5 (roaming).

4. Define and activate the PDP context. AT+CGDCONT=1,"IP","<apn>", then AT+CGATT=1 to attach and AT+CGACT=1,1 to activate. Verify an assigned IP with AT+CGPADDR.

5. Open the socket with the correct vendor stack for the module family.

   • Quectel (EC25/BG96/BC66): AT+QIOPEN, then AT+QISEND/AT+QIRD/AT+QICLOSE.
   • SIMCom 2G (SIM800-class): AT+CIPSTART, then AT+CIPSEND (write payload after >, terminate with Ctrl+Z 0x1A).
   • SIMCom LTE (SIM7600/A76XX-class): AT+NETOPEN first, then AT+CIPOPEN to open a socket and AT+CIPSEND to send (AT+CIPSEND=<link>,<len> takes an explicit length; no Ctrl+Z when a length is given). Read buffered data with AT+CIPRXGET; close with AT+CIPCLOSE/AT+NETCLOSE.
   • u-blox (SARA): AT+USOCR to create, AT+USOCO to connect, AT+USOWR to write, AT+USORD to read, AT+USOCL to close.

6. Build a URC-aware parser. Run a line-based state machine that classifies each line as echo, intermediate (>), final (OK/ERROR/+CME ERROR), or URC (RING, +CMTI:, +QIURC:), and route URCs to callbacks instead of the command waiter.

Common Failures

• Sending AT before boot completes; commands are dropped until the RDY URC or a fixed power-on delay elapses.
• Checking AT+CREG? on an LTE-only module that only registers via +CEREG, so registration looks stuck at 0,2.
• Registration fails from wrong APN, disabled radio (AT+CFUN=0), locked SIM, or poor signal (+CSQ: 99,99).
• Using the wrong socket stack: sending AT+CIPSTART to a SIM7600/A76XX module (which needs AT+NETOPEN+AT+CIPOPEN), or forgetting AT+NETOPEN before AT+CIPOPEN.
• Fixed read timeout truncates variable-length responses like AT+QIRD or a slow AT+COPS=? scan.
• URC (e.g. +QIURC: "recv") arrives mid-response and the parser mistakes it for the command reply, causing a parse race.
• AT+CIPSEND on a SIM800-class module sent without the Ctrl+Z terminator, so the modem waits forever at the > prompt.
• After AT+CPSMS/AT+CEDRXS the module sleeps and drops the socket; sending during PSM fails until wake and re-attach.

Verification

Before claiming the link works:

• State the module, technology, APN, and which registration command applies.
• Confirm +CPIN: READY, a valid +CSQ, and <stat>=1 or 5 on the correct CxREG.
• Confirm AT+CGPADDR shows an IP before opening any socket.
• Send a round-trip TCP payload and confirm the receive path for the module family (+QIURC: "recv"/AT+QIRD, or +CIPRXGET, or +USORD).
• Prove the URC parser handles an unsolicited RING/+CMTI arriving during a command without desync.
• If PSM/eDRX is enabled, confirm wake, re-registration, and data resume.

Example

User:

BG96 用 AT+CEREG? 一直返回 0,2,连不上网,该怎么查?

Agent:

1. Confirms boot done, ATE0 set, then checks AT+CPIN? = READY and AT+CSQ is not 99.
2. Verifies AT+CFUN=1 and the APN via AT+CGDCONT?; fixes APN if wrong for the IoT SIM.
3. Waits for +CEREG: 0,1/0,5, then runs AT+CGACT=1,1 and AT+CGPADDR before opening a socket with AT+QIOPEN.