EG25-GC Mini PCIe SIM not detected - USIM_VDD drops to 0V after 1.8V/3.0V attempt

IoT Modules LTE Standard Module
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1

Hello everyone,

I am designing a custom carrier board for the EG25-GC Mini PCIe module and
facing a persistent SIM detection issue. I hope someone from the community
or Quectel team can help identify the root cause.

MODULE DETAILS:

  • Module: EG25-GC Mini PCIe
  • Firmware: EG25GGCR07A02M1G_30.200.30.200
  • IMEI: 864081066248761
  • Host: Raspberry Pi CM5 (Linux)

ISSUE:
AT+CPIN? always returns +CME ERROR: SIM not inserted
Tried multiple different SIM cards - same result on all

HARDWARE CIRCUIT:

  • SIM socket: A-MicroSIM-1.35B (8-pin with CD pin)
  • USIM_PRESENCE pull-up: R107 51kΩ to +1.8V
  • USIM_VDD pull-up: R6 15kΩ
  • Added 15kΩ between USIM_VDD and USIM_DATA
  • TVS protection: TPD6E05U06RVZR (0.5pF - well below 15pF limit)
  • 33pF caps on CLK, RST, DATA lines
  • 100nF decoupling on SIM VCC

HARDWARE MEASUREMENTS:

  • USIM_PRESENCE with SIM inserted: 1.2V (expected 1.8V - why only 1.2V?)
  • USIM_PRESENCE with SIM removed: 0V
  • USIM_VDD sequence: 1.8V (1 sec) → 3.0V (1 sec) → 0V (stays)
  • SIM never responds to ATR at either voltage

AT COMMANDS TRIED:
AT+QSIMDET=0,0 → SIM still not detected
AT+QSIMDET=1,0 → SIM still not detected
AT+QSIMDET=1,1 → SIM still not detected
AT+QSIMVOL=0 → Auto voltage - not working
AT+QSIMVOL=1 → Force 1.8V - not working
AT+QSIMVOL=2 → Force 3.0V - not working
AT+QDSIM=0 → External slot - confirmed
AT+QDSIM=1 → Slot 1 - not working
AT+QCFG=“sim/onchip”,0 → External SIM confirmed
AT+QPRTPARA=3 → Deep NV restore - no change
AT&F → Factory reset - no change
AT+QINISTAT → Always returns 0 (never initializes)

GPIO PINS (all set correctly on host CM5):

  • PERST# → OUTPUT HIGH :white_check_mark:
  • W_DISABLE# → OUTPUT HIGH :white_check_mark:
  • DTR → OUTPUT HIGH :white_check_mark:

QUESTIONS:

  1. Why is USIM_PRESENCE measuring 1.2V instead of expected 1.8V?
    Could this low voltage be preventing SIM detection even with
    AT+QSIMDET=1,1 (HIGH level detection)?

  2. Is there a minimum voltage threshold for USIM_PRESENCE to be
    recognized as HIGH by the EG25-GC?

  3. Could the USIM_VDD sequence (1.8V→3.0V→0V) indicate the module
    IS trying to power the SIM but the SIM is not responding?
    Or does it mean the module is not detecting USIM_PRESENCE correctly?

  4. Is there any known issue with firmware EG25GGCR07A02M1G_30.200.30.200
    related to SIM detection? Would updating to latest firmware help?

  5. Could the EG25-GC module itself be damaged? How can I confirm this?

I can provide schematic sections if needed.

Thank you very much for your help

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p!

2

Dear Customer,

Hi, thanks for the very detailed report, the measurements you captured are exactly what’s needed to narrow this down. Based on your data, there are actually two separate issues stacked together, and both need to be addressed to get a valid ATR.

The most important clue is your USIM_VDD sequence: 1.8V (1s) → 3.0V (1s) → 0V. This is the standard ISO 7816-3 activation attempt. The module applies VCC at 1.8V (class C), releases CLK/RST and waits for an ATR, gets no response, retries at 3.0V (class B), still gets nothing, then deactivates. This tells us two things:

  1. The module’s SIM engine is working correctly — it powers, clocks and cold-resets the card as expected. The module is very likely NOT damaged.
  2. Even so, the card is not returning an ATR. When you ran AT+QSIMDET=0,0, presence detection was disabled and the module attempted activation regardless of the CD pin — yet still no ATR. That points to a real signal/contact problem on the SIM interface itself, independent of the presence-detect issue.

So there are two problems: (A) marginal presence detection (your 1.2V reading), and (B) no ATR even when detection is bypassed. Problem B is the one actually blocking AT+CPIN?.

Answers to your questions:

Q1 & Q2 (Why 1.2V, and is there a threshold?): Yes, 1.2V is a genuine problem. USIM_PRESENCE is a 1.8V-referenced logic input, so V_IH is roughly 0.65–0.7 × 1.8V ≈ 1.17–1.26V. Your 1.2V sits right on that edge and is very likely being read as LOW (“no card”). The cause is R107 = 51kΩ being too weak: with the card inserted and the CD switch open, that 51kΩ forms a divider against the module pin’s internal pull-down (~100kΩ), landing you around 1.2V, which matches your measurement. Recommended fix: reduce R107 to about 4.7kΩ–10kΩ so the inserted level rises to ~1.7–1.8V, comfortably above V_IH. With that corrected, AT+QSIMDET=1,1 is the right polarity for your wiring (1,0 inverts the logic and will always fail).

Q3 (Does the VDD sequence mean the module is trying but the SIM isn’t responding?): Yes, exactly. Since you observed the sequence with detection disabled, presence was already out of the loop. It means the module powered and reset the card and received nothing back — a physical SIM-link problem (missing/swapped signal, bad contact, or grounding issue at the socket).

Q4 (Firmware / known issue?): No. EG25GGCR07A02M1G is a standard build with no SIM-detection defect that produces this. New firmware will not fix wiring or contact issues, so I would not expect an update to resolve it. Keep firmware current as good practice only.

Q5 (Is the module damaged?): Very unlikely. It enumerates, responds to AT, and runs the full activation state machine. The decisive test is to place this exact module on a Quectel EVB or a known-good reference carrier and insert the same SIM. If it reads the card there, the module is fine and the fault is on the custom board.

Recommended next steps, in order:

  1. Power-off continuity check from each socket contact to the module pin, for VCC, CLK, RST, I/O (DATA) and GND. On a new board, a swapped CLK/RST or a floating card GND is the single most common cause of “activates but no ATR.” Verify against the A-MicroSIM-1.35B datasheet pinout, not the silkscreen.

  2. Scope VCC, CLK, RST and I/O at the card contacts during an activation attempt. You should see VCC reach 1.8/3.0V at the card, a CLK burst (~3.25MHz), RST go high, then look for the ATR on I/O. Whichever signal is missing at the pad is the culprit.

  3. Check the socket mechanically — micro-SIM sockets with CD often have marginal spring contact or seating. Try reseating, a fresh socket, or reflowing it.

  4. Reduce R107 to ~4.7–10kΩ, re-measure USIM_PRESENCE inserted (should be ~1.7–1.8V), then confirm AT+QSIMDET=1,1 tracks card insertion/removal correctly.

  5. Sanity-check the 33pF caps. They are right at the recommended line-capacitance limit. If RST/CLK edges look slow on the scope, temporarily reduce them to 10–22pF (or lift them) to rule out edge degradation.

  6. Confirm the USIM_VDD LDO can source the momentary SIM current, and that the 100nF (ideally plus a bulk cap) is placed at the card.

The 15kΩ VDD-to-DATA pull-up and the TPD6E05U06 (0.5pF) are both fine — neither is the problem.

My expectation is that step 1 or 2 will reveal a swapped or non-contacting signal at the socket. Once you scope the four SIM signals at the card contacts during activation, you will know exactly which line is dead. Feel free to post the scope captures and I can help interpret them.

Best regards