# How to enable SIM1 interface on GC02S1 ? AT+CSUS=1 returns ERROR

**URL:** <https://forum.sequans.com/t/how-to-enable-sim1-interface-on-gc02s1-at-csus-1-returns-error/653>\
**Category:** Miscellaneous\
**Created:** [October 9, 2026, 9:33am UTC](https://forum.sequans.com/t/how-to-enable-sim1-interface-on-gc02s1-at-csus-1-returns-error/653 "2026-10-09T09:33:26Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![DrCooki](https://dub1.discourse-cdn.com/flex018/user_avatar/forum.sequans.com/drcooki/32/109_2.png) [@DrCooki](https://forum.sequans.com/u/DrCooki)\
**Post date:** [October 9, 2026, 9:33am UTC](https://forum.sequans.com/t/how-to-enable-sim1-interface-on-gc02s1-at-csus-1-returns-error/653/1 "2026-10-09T09:33:26Z")

</div>

Hi,

We use GC02S1 module on our board and we have connected both SIM0 and SIM1 interfaces. According to the online AT command reference manual at [AT Commands Reference Manual | LR90 Rev. 3](https://docs-online.sequans.io/GC02S1/AT_Command/#AT_CSUS) we must use AT+CSUS to enable SIM1 interface. But AT+CSUS=1 returns ERROR.

See the log below:

ATI0  
SEQUANS Communications  
GC02S1-NA2  
UE9.0.6.1  
OK

ATI1  
UE9.0.6.1  
LR9.0.6.1-64123  
OK

AT+CFUN=0  
OK

AT+CSUS?  
+CSUS: 0  
OK

AT+CSUS=0  
OK

AT+CSUS=1  
ERROR

Thank you for your help.

---

<div class="post-metadata">

**Author:** ![Peng\_Gao](https://avatars.discourse-cdn.com/v4/letter/p/45deac/32.png) [@Peng\_Gao](https://forum.sequans.com/u/Peng_Gao)\
**Post date:** [October 9, 2026, 2:42pm UTC](https://forum.sequans.com/t/how-to-enable-sim1-interface-on-gc02s1-at-csus-1-returns-error/653/2 "2026-10-09T14:42:45Z")

</div>

Enabling SIM1 on a GC02S1 module has two parts: first enable the interface at hardware level with `AT+SQNHWCFG`, then select it as the active SIM with `AT+CSUS`. By default SIM0 is enabled and SIM1 is disabled, and SIM1 shares its pins with GPIO25, GPIO26 and GPIO27.

**Step 1: Enter manufacturing mode**

```auto
AT+CFUN=5
OK

```

**Step 2: Check the current SIM1 configuration (optional)**

```auto
AT+SQNHWCFG="sim1"
+SQNHWCFG: sim1_data: disable
+SQNHWCFG: sim1_clk: disable
+SQNHWCFG: sim1_resetn: disable
+SQNHWCFG: sim1: disable
+SQNHWCFG: sim1_pollinginterval: 0
OK

```

**Step 3: Free the shared GPIO pins**

```auto
AT+SQNHWCFG="gpio25","disable"
AT+SQNHWCFG="gpio26","disable"
AT+SQNHWCFG="gpio27","disable"

```

If you skip this step, enabling SIM1 fails with `sim1_data BUSY (gpio25)` and similar errors for the other two pins.

**Step 4: Enable SIM1 with software polling**

```auto
AT+SQNHWCFG="sim1","enable","10000"
OK

```

SIM1 has no SIM-detect pin, so you must give a polling interval. Without one, the command returns `ERROR: sim1_detect UNSUPPORTED`. The interval can be 1000 to 30000 ms, and 10000 ms is the recommended value. Don’t use values below 1000. Polling is paused while the module is in low power mode.

**Step 5: Reboot so the change takes effect**

```auto
AT^RESET

```

The setting is saved permanently and survives reboots and firmware upgrades.

**Step 6: Select SIM1 as the active slot**  
The modem must be in `CFUN=0` for this:

```auto
AT+CFUN=0
AT+CSUS=? → +CSUS: 2 (two slots available)
AT+CSUS=1 → select SIM1
AT+CSUS? → +CSUS: 1
AT+CFUN=4 → airplane mode so the SIM gets read
AT+CIMI → check that the IMSI belongs to the SIM1 card

```

**Hardware notes from the GM02S datasheet:**

- SIM1 is mainly intended for soldered SIM chips.
- SIM1 has no VCC output, so the SIM needs its own external 1.8 V supply. If you only need one SIM, Sequans recommends using SIM0 instead.
- The pins are GPIO25/SIM1\_IO (pad 39), GPIO26/SIM1\_CLK (pad 40) and GPIO27/SIM1\_RESETN (pad 41).