IoT Fire Alarm Device Design
Thought for 8 sec
You should ask Quectel / supplier for the EC25MXGA hardware design package, not just the marketing datasheet.
Ask for:
-
EC25MXGA Hardware Design Guide
- Full pin descriptions
- Power supply requirements
- PWRKEY / RESET timing
- UART / USB / SIM / RF reference circuits
- VDD_SDIO usage guidance
-
Recommended PCB land pattern / footprint
- Pad dimensions
- Pad numbering
- Paste mask / solder mask recommendations
- Keepout zones under/around the module
- Assembly notes
-
Reference schematic
- VBAT_RF / VBAT_BB decoupling
- SIM socket wiring
- Antenna matching / ESD
- Status pins
- Power-on/off sequence
-
RF layout guidelines
- 50 Ω antenna routing
- Ground stitching via recommendations
- Antenna connector/matching network placement
- Diversity/GNSS pins if used or unused
-
Certification documents
- RCM / ACMA / AS/CA S042 relevance
- Carrier approval info for Australia
- LTE band support for AU carriers
- Test reports if available
-
Lifecycle / supply confirmation
- Active production status
- Recommended replacement if EC25MXGA is not preferred
- Minimum order / lead time
Short message you can send:
We are designing a dual-modem Australian fire alarm signalling device using 2 × EC25MXGA. Please provide the EC25MXGA hardware design guide, recommended PCB land pattern/footprint, reference schematic, RF layout guide, SIM interface recommendations, VDD_SDIO connection guidance, Australian carrier/certification documentation, and lifecycle/supply status.
Ok I’ll contact them
Thought for 14 sec
Good plan. Ask them specifically for the hardware design guide + recommended PCB land pattern for EC25MXGA-128-SNNS.
Until we get that, the safest path is:
- Keep EC25MXGA in the design.
- Do not send the PCB to manufacture yet.
- Continue with placement/routing cleanup only as a draft.
- Finalize modem footprint, SIM wiring, RF layout, and
VDD_SDIOtreatment once Quectel confirms the official guidance.
Clean draft layout
Review EC25 wiring
Check power budget
Update factory checklist
Run current checks
Refine this doc
Ask about this doc
Protected 12–24 VDC Power Architecture
Goal
Design the incoming power and rail architecture for an ASE-style fire alarm signalling device with both Quectel cellular modems active at all times.
Key source assumptions
- Input supply target from user: 12–24 VDC.
- Centaur-style reference unit accepts approximately 9.5–29 VDC; our design should tolerate at least this range if possible, while detecting incorrect voltage.
- Quectel EC21 VBAT requirement from Quectel hardware design references:
- VBAT operating range: 3.3–4.3 V, 3.8 V typical.
- Peak transmit current: up to approximately 2.0 A per modem.
- Voltage drop during burst transmission should be limited to approximately 400 mV.
- STM32H757 family VDD: 1.62–3.6 V, normally use 3.3 V I/O rail with supervisor.
Design requirement
Because both SIM/modem paths are always on and polling/checking in every 20 s, the power system must be sized for simultaneous modem transmit peaks, not only average power.
Recommended input protection chain
12–24 VDC field input terminal:
- Replaceable fuse or resettable eFuse / electronic fuse.
- Reverse-polarity protection:
- Preferred for production: ideal-diode PMOS/NMOS controller or MOSFET reverse protector for low heat and low voltage drop.
- Simpler fallback: series Schottky diode, but check heat at full modem current.
- Surge/transient protection:
- TVS diode selected for 24 V systems and expected fire-panel transients.
- Input capacitor and damping as needed.
- EMI/noise filter:
- Common-mode/differential filtering as required by EMC testing.
- LC or ferrite + bulk capacitance before converters.
- Over-voltage cutoff / incorrect voltage protection:
- Disconnect load if input exceeds safe limit.
- Target cutoff to be finalized; initial design target: survive 29 V continuous and disconnect above approximately 30–33 V.
- Under-voltage / brownout detection:
- Detect low input and report power trouble before rails collapse.
- Initial warning threshold around 10.5–11 V for 12 V systems; exact threshold depends on fire-panel battery behavior.
Rail plan
Recommended rails:
- VIN_PROTECTED: protected 12–24 V domain after input protection.
- 4V0_MODEM_A: high-current rail for Quectel modem 1.
- 4V0_MODEM_B: high-current rail for Quectel modem 2.
- 3V3_SYS: STM32, logic, buttons, Dallas/iButton, SPI memory, LCD logic.
- 3V0_SIM_A / 1V8_SIM_A if required: SIM supply controlled by modem 1 per Quectel reference design.
- 3V0_SIM_B / 1V8_SIM_B if required: SIM supply controlled by modem 2 per Quectel reference design.
- 5V_LCD or LED backlight rail if selected LCD requires it.
- RELAY_COIL rail: 12/24 V or regulated rail depending on relay selection.
- FAS_INPUT_EXCITE / ADC reference: stable measurement/excitation domain for supervised input voltage decoding.
Modem rail sizing
Each EC21 rail should support:
- 3.8–4.0 V output target.
- At least 2.0 A peak current per modem.
- Recommended design margin: 2.5–3.0 A peak capability per modem rail.
- Local bulk capacitance near VBAT pins per Quectel hardware guide.
- Separate power switch/current limit per modem.
- Independent voltage/current monitoring where possible.
For both modems active:
- Peak modem current budget: 4.0 A minimum combined at ~3.8 V.
- Recommended converter capability: at least 5–6 A combined modem power capability or two independent 3 A modem bucks.
- At 12 V input, modem peak input current estimate before losses: about 1.3 A for both modems at 3.8 V × 2 A × 2. With converter losses and margin, size upstream protection for higher transient current.
Logic rail sizing
Initial 3.3 V rail budget:
- STM32H757: allow at least 250–500 mA worst-case depending clock/peripherals/temp.
- LCD logic/backlight: depends on selected display; keep backlight on separate rail or switch.
- SPI memory, FRAM/flash, buttons, Dallas key, sensors: modest but include margin.
- Recommended 3V3_SYS regulator: at least 1 A, preferably 1.5–2 A if LCD logic or peripherals share the rail.
Monitoring and firmware-visible signals
The STM32 should monitor/log:
- VIN_PROTECTED voltage.
- Input overvoltage/undervoltage fault state.
- 3V3_SYS power-good.
- MODEM_A rail power-good and optional current/overcurrent fault.
- MODEM_B rail power-good and optional current/overcurrent fault.
- Relay supply present.
- FIP/power-fail input if provided by panel.
Safety/reliability decisions
- Do not let a short/fault on one modem rail collapse the MCU or the second modem.
- Modem A and modem B should have separate current limiting and power-control FETs/load switches.
- MCU should be able to hard-cycle each modem independently using PWRKEY/RESET and rail enable.
- Preserve event logging during brownout as long as possible; consider hold-up capacitance or early power-fail interrupt.
Open choices before schematic
- Choose exact reverse-protection topology: simple diode vs ideal-diode/MOSFET.
- Choose exact over-voltage cutoff threshold.
- Choose whether the two modem rails use two separate bucks or one high-current buck plus two protected load switches. Separate bucks are preferred for fault isolation.
- Select relay coil voltage/contact rating.
- Select LCD/backlight power requirements.
Recommended baseline for schematic
Use this baseline unless changed:
- 12–24 VDC input terminal.
- eFuse or fuse + ideal-diode reverse protection.
- 33 V-class overvoltage cutoff / 24 V-system TVS selected during component choice.
- Two independent 4.0 V, 3 A buck rails for Quectel modems.
- One 3.3 V, 1.5–2 A logic buck/LDO chain for STM32 and logic.
- Separate switchable LCD backlight rail.
- Per-rail power-good and fault signals to STM32.
Goal
Key source assumptions
Design requirement
Recommended input protection chain
Rail plan
Modem rail sizing
Logic rail sizing
Monitoring and firmware-visible signals
Safety/reliability decisions
Open choices before schematic
Recommended baseline for schematic
