The HC-12 module is a half-duplex 100mW multi-channel FM transceiver for digital data transmission with a built-in microcontroller that allows it to run autonomously without external microcontrollers.
Its primary use case is wirelessly transmitting serial data at various baud rates.
Note that HC-12 is capable of outputting RF signals ten times above the legal limit of license-free ISM bands (100 mW instead of 10 mW). If you are not a licensed radio amateur, you must program the board to emit 10 mW at maximum.
Overview
Sometimes there is a need to have a data connection between devices over some distance. HC-12 works as a wireless UART extender, forwarding the transmitted information wirelessly to another HC-12 module.
To enable this, HC-12 has a built-in microcontroller. Its firmware autonomously takes care of all required tasks and controls the SI4438/SI4463 radio chip via an internal SPI interface.
The user simply connects the UART output of one device to RX and TX, and picks up the signals at the other HC-12 module at its RX and TX pins.
Ease of Use
It is important to understand that HC-12 is a specialized application that does not expose the SI44xx radio chip or its capabilities to the outside. Instead, its built-in microcontroller interfaces with the radio chip.
This makes using this module so simple: to the outside, a user simply connects the data lines. HC-12 immediately starts transferring the data wirelessly.
The user can configure the module by setting certain high-level preferences, such as focussing on low power, high speed, or long distance. HC-12 internally then picks the most appropriate FM modulations and radio settings.
The module covers the frequency range 425-525 MHz and divides it into 100 channels at a 400 kHz spacing, and the user picks a channel.
Limitations
The downside to this abstraction is that you have no access to low level radio settings, and you can transfer data only as UART data streams.
If you want to directly access the SI44xx** API, you would have to re-program the onboard microcontroller as it is the only one with direct *SPI access to the radio module. This is complex, so if you need low-level access to radio functions, HC-12 is not for you. Instead, choose a simpler board without microcontroller and with an exposed SPI interface so you can use your own microcontroller to talk to the radio module.
Built-In Microcontroller
HC-12 uses the 16 MHz 8-bit STM8S003F3 microcontroller made by STMicroelectronics with proprietary firmware that translates a set of UART commands into commands for the internal ST44xx radio chip.
The package type of the microcontroller may vary: both small square-shaped U6 (LQFP-20) or larger P6 (TSSOP-20) package types are commonly found.
Clones: SI4438 vs. SI4463
HC-12 is available in many versions and cloned designs. They all use a SI44xx radio chip from Silicon Labs. Unbranded and very affordable boards often use the cheaper and less feature-rich SI4438.
Both versions are marketed under the same name: HC-12. Vendors often use SI4438 and SI4463 interchangeably in their ads, still shipping SI4438 only. Check the chip markings on the boards you receive.
SI4438 and SI4463 differ considerably:
| Feature | SI4463 | SI4438 |
|---|---|---|
| Power Supply | 1.8 V to 3.6 V | 1.8 V to 3.6 V |
| Frequency Range | 119-1050 MHz | 425-525 MHz |
| Modulation | (G)FSK, 4(G)FSK, (G)MSK, OOK | (G)FSK, OOK |
| Data Rate | 0.123 kbps - 1 Mbps | 100 - 500 kbps |
| Output Power | up to +20 dBm | up to +20 dBm |
| Receiver Sensitivity | -126 dBm | -124 dBm |
| Current Consumption (TX) | 20 mA at +20 dBm | 22 mA at +20 dBm |
| Current Consumption (RX) | 10/13 mA | 14 mA |
| Standby Consumption | 50 nA | 50 nA |
| Integrated Features | AES encryption | Low-power sleep mode |
From a practical perspective, these are the relevant consequences:
- Compatiblity: make sure not to mix boards with SI4438 and SI4463. To leverage features of the SI4463, the firmware needs to use modulations that are not available in SI4438, making both incompatible.
- Recommendation: if you have the choice, opt for boards with SI4463 as it is the far superior chip with a much better sensitivity (boosting communication distance by 25%) and up to 30% better battery life in receive mode.
The UART command AT+VER returns the board versions. “Original” boards should return www.hc01.com (and have this URL printed on their back side). That said, clone boards from other sources may work reliably, too.
Connectors
Antenna
One side of the board features a through-hole connector and an IPEX connector, both for attaching the antenna. The antenna solder pin is flanked by two GND pins on either side.
Always operate this device with an antenna. If you don’t have a designated antenna, a simple 17.3 cm wire will do. Operating without an antenna can irreversibly damage the RF unit.
Interface
On the opposite end, five through-holes provide connectivity, clearly marked on the backside:
| Pin | Tag | Description |
|---|---|---|
| 1 | SET | pulled up, connect to GND to issue AT commands via serial |
| 2 | TXD | Serial interface |
| 3 | RXD | Serial interface |
| 4 | GND | Ground |
| 5 | VCC | 3.2 - 5.5V |
The LDO voltage regulator on this board is problematic. When operating at maximum power over extended periods, the LDO needs to convert considerable energy to heat in an effort to lower the input voltage to the required level. To workaround, lower the input voltage yourself to 4.5 V. This can also be achieved by placing a 1N4007 diode in series with a 5 V power supply.
Configuring HC-12
With HC-12, only the built-in microcontroller has direct access to the radio module. The user cannot directly control them and uses simple AT commands instead to tell the built-in microcontroller what to do:
- Priorities:
a choice of four different transmission modes (FU1-FU4) allow the user to prioritize power consumption, speed, or communication distance. Internally, the microcontroller translates these modes into the most appropriate FM modulations and radio settings. - Channel:
the user cannot set a specific frequency. Instead, there is a choice of 100 channels. Internally, the microcontroller translates these channels into frequencies within the 425-525 MHz frequency range. Only channels 1-4 represent license-free frequencies in Europe. - Power:
the user can set the RF output power in eight steps from 0.8 mW to 100 mW, roughly doubling the power with each step. Only the first four RF power choices (0.8 mW, 1.6 mW, 3.2 mW, 6.3 mW) are legal to use in license-free bands. - Serial Interface:
the user can configure speed and settings of the serial interface so it matches the devices that need to communicate. HC-12 supports baud rates in the range of 1200-115200. However, the maximum baud rate depends on the transmission mode you chose and is limited to 4800 inFU2and to 1200 inFU4.FU1andFU3do not restrict the available baud rates. In addition, number of bits, parity, and stop bits can be defined.
By default, HC-12 uses FU3 at 9600,8,N,1 on channel 1 (433.4MHz).
Sending Commands
By default, HC-12 works in data mode and treats all input at RX as data that is sent off.
To instruct the microcontroller to treat input as commands, pin SET must be connected to GND. While this connection exists, all input is treated as AT commands.
To reset the communication settings to the default 9600 baud, 8, N, 1, connect SET to GND, then power on HC-12.
Choosing Transmission Mode
The transmission mode prioritizes low power consumption, speed, or communication distance.
The mode is set via AT+FU1 through AT+FU4.
| Mode | FU1 |
FU2 |
FU3 |
FU4 |
|---|---|---|---|---|
| Use Case | power efficient | extreme low power | normal | long distance |
| Power (receive) | 3.6 mA | 80 uA | 16 mA | 16 mA |
| Latency | 15-25 ms | 500 ms | 4-80 ms | 1000 ms |
| Maximum baud rate | 115.200 | 4.800 | 115.200 | 1.200 |
| Communication Distance (100 mW) | 100 m | 100 m | 1.000 m (2.400 baud) 600 m (9.600 baud) |
1.800 m |
| Air Baud Rate (kbps) | 250 | 250 | 236 | 0.5 |
- In
FU2, receiving power consumption is extremely low (80 uA). The price you pay speed capped at 4.800 baud, plus long latency (500 ms): the radio chip is sent to sleep in regular intervals. - The sleep current is 22uA (in sleep, the board is not operational and cannot receive).
- When transmitting with full power, power consumption can raise to 100 mA, regardless of mode.
Choosing Baud Rate
The baud rate at which the data is delivered by the external device can be set via AT+B*x* (where x is the baud rate, i.e. AT+B9600).
| Serial Baud Rate | 1.200 | 2.400 | 4.800 | 9.600 | 19.200 | 38.400 | 57.600 | 115.200 |
|---|---|---|---|---|---|---|---|---|
| Air Baud Rate | 5.000 | 5.000 | 15.000 | 15.000 | 58.000 | 58.000 | 236.000 | 236.000 |
FU1 |
yes | yes | yes | yes | yes | yes | yes | yes |
FU2 |
yes | yes | yes | - | - | - | - | - |
FU3 |
yes | yes | yes | yes | yes | yes | yes | yes |
FU4 |
yes | - | - | - | - | - | - | - |
| RX Sensitivity (dBm) | -117 | -117 | -112 | -112 | -107 | -107 | -100 | -100 |
- The trade-off is speed versus communication distance because with faster data transfer rates, the receiver sensitivity decreases. That’s why the long range mode
FU4supports only slow 1.200 baud. - Due to error correction and related overhead, the data transmission rate (serial baud rate) is always considerably lower than the air baud rate.
RF Power
The radio chip on HC-12 boards supports RF power of up to 20dBm/100mW and can be controlled in eight steps via AT+P1 through AT+P8.
The default is maximum RF power at 100 mW.
| Power Level | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
|---|---|---|---|---|---|---|---|---|
| mW | 0.8 | 1.6 | 3.2 | 6.3 | 12 | 25 | 50 | 100 |
| dBm | -1 | 2 | 5 | 8 | 11 | 14 | 17 | 20 |
In most regions of the world, the limit for the license-free 433 MHz band is 10 mW (power levels 1-4). For RF power exceeding 10 mW, a license is required (i.e. HAM Radio Amateur).
Channel
With AT+C1 through AT+C100 the user can select a channel. HC-12 translates channels to frequencies, starting at 433.4 MHz, with a spacing of 400 kHz. So channel #1 represents 433.4 MHz, channel #2 433.8 MHz, channel #3 434.2 MHz, and so forth.
Channel #1 is set by default.
In Europe, the license-free frequency range is 433.05-434.79 MHz, so only channels #1-#4 are legal to use (without a license).
Incompatibilities
Internally, the firmware translates the selected operation mode into appropriate digital FM modulations.
This explains reports that HC-12 modules from different vendors cannot communicate. If you experience issues like this, start by checking the firmware versions using AT+V.
If firmware versions do not match, they may use different FM modulations for the more specialized modes FU1, FU2, and FU4. Try FU3 to see whether the modules can connect now.
One of the reasons why different firmware versions exist that use incompatible radio settings lays in the fact that HC-12 boards use different radio chips.
Boards with the more sophisticated SI4463 take advantage of more sophisticated modulations like (G)MSK, which increases data rates and/or long distance communication. Since these modulations aren’t supported with boards using the SI4438, the result are incompatibilities.
Communication distance reduces by half for every 6dBm decrease in receiver sensitivity.
Materials
HC-12 Manual
Silicon Lab Si4438 Datasheet
Silicon Lab Si4463 Datasheet
STM8S103 Microcontroller Datasheet
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(content created Apr 01, 2024 - last updated Mar 22, 2025)