KSP-N51U Network Attach
The KSP-N51U runs Nordic Semiconductor's Serial Modem firmware and is controlled over UART with AT commands. It is set up to attach to the cellular network automatically at power-on — for most users there is nothing to configure.
Most of the time you don't need this page — power the module and wait, and it attaches on its own. Come here only for the exceptions: to confirm the attach, to switch the radio mode (LTE-M vs NB-IoT), or to attach manually with a custom APN.
Attach Flow
| Step | Action | Command | Expected Result |
|---|---|---|---|
| 1 | Power on the module | — | Module attaches automatically |
| 2 | Confirm registration | AT+CEREG? | Status 1 (home) or 5 (roaming) |
| 3 | Check signal | AT+CESQ | RSRP better than −110 dBm |
| 4 | Confirm data connection | AT+CGPADDR | Module has an assigned IP address |
Steps 2–4 are checks, not configuration. You only need the manual attach section for uncommon situations such as a custom APN or a specific radio mode.
How Attach Works by Default
At power-on the module brings up the radio and registers on the network on its own. Power it up, wait up to about a minute for band scanning, and it comes online — no AT commands required.
Confirm the Module Attached
1. Check Registration Status
This is the check that matters most. Query the network registration state:
AT+CEREG?
+CEREG: 1,1
OK
The second value is the registration status:
+CEREG status | Meaning |
|---|---|
1 | ✅ Registered — home network |
5 | ✅ Registered — roaming |
2 | Still searching — wait a moment |
0 / 3 / 4 | Not attached — see manual attach below |
2. Check Signal and IP
AT+CESQ
+CESQ: 99,99,255,255,20,54
OK
AT+CGPADDR
+CGPADDR: 0,"10.0.0.1"
OK
Aim for RSRP better than −110 dBm. An assigned IP from AT+CGPADDR confirms the data connection is up.
Set the Radio Mode (LTE-M or NB-IoT)
This is the setting most users configure. Use AT%XSYSTEMMODE to choose which radio access technology the module uses:
AT%XSYSTEMMODE=<LTE-M>,<NB-IoT>,<GNSS>,<preference>
| Parameter | Meaning | Value |
|---|---|---|
<LTE-M> | Enable LTE-M | 1 = on, 0 = off |
<NB-IoT> | Enable NB-IoT | 1 = on, 0 = off |
<GNSS> | Enable GNSS | 1 = on, 0 = off |
<preference> | Which to prefer when both are on | 0 none · 1 LTE-M · 2 NB-IoT |
Common presets:
AT%XSYSTEMMODE=1,1,0,1 LTE-M + NB-IoT, prefer LTE-M (typical)
AT%XSYSTEMMODE=1,0,0,0 LTE-M only
AT%XSYSTEMMODE=0,1,0,0 NB-IoT only
Change the system mode with the radio off, then bring it back online:
AT+CFUN=4 Go offline
AT%XSYSTEMMODE=1,1,0,1 Set the radio mode
AT+CFUN=1 Go online; attach begins
Manual Attach
Use this only if your SIM needs an APN different from the default, or the module did not attach on its own.
1. Go Offline and Configure
AT+CFUN=4 Go offline
AT%XSYSTEMMODE=1,1,0,1 Choose the radio mode
AT+CGDCONT=0,"IP","<your-apn>" Set the APN
2. Go Online and Attach
AT+CFUN=1 Go online; attach begins
AT+CEREG=5 Subscribe to registration updates
AT+CEREG? Poll until status is 1 or 5
3. Verify the Checkpoints
| Checkpoint | Command | Pass Condition |
|---|---|---|
| Registered | AT+CEREG? | status 1 or 5 |
| Signal OK | AT+CESQ | RSRP better than −110 dBm |
| Data up | AT+CGPADDR | non-empty IP address |
Troubleshooting
| Symptom | Likely Cause | Recommended Action |
|---|---|---|
Stuck at +CEREG: ...,2 | Weak coverage / band not supported | Check antenna; confirm operator LTE-M / NB-IoT support |
+CEREG: ...,3 (denied) | SIM not provisioned / wrong APN | Verify data plan; set the APN manually |
| Wrong radio mode | Operator only supports one RAT | Set AT%XSYSTEMMODE for LTE-M only or NB-IoT only |
| Registered but no IP | PDP context inactive | Re-check the APN; confirm the plan allows data |
For registration error codes in depth, see the FAQ.
Related Resources
- Quick Start — hardware bring-up and first AT command
- AT Commands — full command reference
- FAQ — registration troubleshooting