ModBee Hat – CAT (SIM7070G)

Low-power cellular hat for the ModBee Node-UIO, built around the SIMCom SIM7070G. It adds LTE Cat-M1 and NB-IoT with GPRS/EDGE fallback, plus GNSS positioning, for sites without Wi-Fi.

The Node-UIO talks to the module with AT commands over UART0. A USB-C port on the hat gives a PC direct access to the module.

Module

Figures from SIMCom's SIM7070G product page.

Network and band support depends on your carrier and country. Check that your SIM and carrier support Cat-M1 or NB-IoT on the bands above. Some carriers no longer run 2G.

Board features

• Power: GPIO9 high switches the hat's 3.3 V rail on through a P-channel MOSFET. JP1 bypasses the switch. A buck-boost converter (U1) makes the module's 3.6 V supply from that rail.
• Power key: GPIO10 drives the module's PWRKEY through a transistor. GPIO10 high pulls PWRKEY low. The PWR button (SW1) does the same by hand.
• UART: the module's UART1 connects to the Node-UIO's U0RX/U0TX (GPIO44/GPIO43) through MOSFET level shifters, 3.3 V to 1.8 V.
• USB-C (USB 2.0) connects straight to the module's USB, with ESD protection. Use it for AT commands from a PC and for module firmware updates. USB doesn't power the module, so the hat must be powered from the Node-UIO.
• BOOT button (SW2) pulls BOOT_CFG high for USB firmware download.
• Nano SIM holder.
• Antennas: two IPEX (U.FL) connectors, LTE and GPS. The GPS connector supplies 3.3 V bias for an active GNSS antenna.
• LEDs: PWR (hat powered), STS (module STATUS), LTE (module NETLIGHT).

The module's I2C, ADC, PCM, GPIO and extra UARTs are not connected on this hat.

Connections

The hat headers J1 and J2 follow the common hat connector pinout. This hat uses:

Only one cellular hat can be fitted. The CAT and LTE hats both use UART0 and GPIO10.

Power key and LEDs

PWRKEY works like a phone's power button: press it to turn the module on, and press it again for longer to turn it off. Timings from SIMCom's SIM7070 series documentation:

• On: hold PWRKEY low (GPIO10 high) for about 1 s. Wait at least 1 s after the hat powers up before you press it. The module is ready in about 3–4 s.
• Off: hold it low for at least 1.2 s, or send AT+CPOWD=1.
• Never hold it low for 12 s or more. That resets the module.

The NETLIGHT pattern can be changed with AT+SLEDS.

Getting started

1. With power off, insert a nano SIM and connect the LTE antenna, plus the GPS antenna if you need GNSS.
2. Fit the hat to the Node-UIO with stacking headers.
3. Power the hat and press PWRKEY from your sketch:

#include <Arduino.h>

constexpr int HAT_POWER = 9;   // high = hat powered
constexpr int PWRKEY    = 10;  // high = PWRKEY pressed
constexpr int MODEM_RX  = 44;  // U0RX
constexpr int MODEM_TX  = 43;  // U0TX

// UART0. Log over native USB so UART0 is free for the modem.
HardwareSerial modem(0);

void setup() {
  pinMode(PWRKEY, OUTPUT);
  digitalWrite(PWRKEY, LOW);
  pinMode(HAT_POWER, OUTPUT);
  digitalWrite(HAT_POWER, HIGH);
  delay(1000);

  digitalWrite(PWRKEY, HIGH);
  delay(1000);
  digitalWrite(PWRKEY, LOW);

  modem.begin(115200, SERIAL_8N1, MODEM_RX, MODEM_TX);
  delay(4000);
  modem.println("AT");
}

4. Check the module answers OK, then set up the network, for example:
   • AT+CPIN?: SIM ready
   • AT+CNMP=38 and AT+CMNB=1: LTE only, Cat-M1 only (AT+CMNB=2 for NB-IoT)
   • AT+CGDCONT=1,"IP","your.apn": set your carrier's APN
   • AT+CSQ, AT+CEREG?: signal strength and registration

The SIM7070 series AT command manual covers TCP/UDP, HTTP(S), MQTT and GNSS (AT+CGNSPWR, AT+CGNSINF).

Documentation

Hardware design and AT command manuals are on the SIM7070G product page. The downloads need a free SIMCom account.

Schematic

schematics/modbee-hat-cat.pdf

Licence

CC BY-SA 4.0