CRx Series Modules

This topic describes information about implementing serial communication between CRx series modules and MCUs, suited for developing smart products such as gateways.

Overview

CRx series is the Wi-Fi and Bluetooth Low Energy (Bluetooth LE) combo module developed by Tuya Smart. It supports both the access point (AP) mode and station (STA) mode for Wi-Fi connection as well as a connection over Bluetooth LE.

Serial communication protocol

Serial Communication Protocol
Serial Communication Protocol for Low Power Consumption
Serial Communication Protocol for HomeKit

Serial communication between a module and an MCU

• Connection between a module and a 3.3V MCU

  

  The VCC pin is the power pin of the module. The TXD and RXD pins are used for serial communication.

• Connection between a module and a 5V MCU

  In the following circuit diagram, voltage level translation can be implemented with a bidirectional voltage-level translator, a MOS transistor, or a triode.

  

  The VCC pin is the power pin of the module. The TXD and RXD pins are used for serial communication.

Level translator reference

• N-channel MOSFET level translator

  In the following circuit diagram, an N-channel MOSFET and a built-in body diode are used to implement two-way communication.

  

• NPN triode level translator

  In the following circuit diagram, an NPN triode is used to implement one-way communication.

  

Design specification

• The following table lists the specification and pin information of CRx modules for serial communication with an MCU.

  Module model	Input voltage (TYP)	Input current (MAX)	Power pin	Power pin on silkscreen	TXD pin	TXD on silkscreen	RXD pin	RXD on silkscreen
  CR3L	3.3V	342 mA	8	VD33	16	TX	15	RX
  CR3L-IPEX	3.3V	342 mA	8	VD33	16	TX	15	RX
  CR2S	3.3V	331 mA	1	3V3	7	TX	5	RX

• Power supply

  • Given that the supply current for 3.3V modules must be greater than the maximum input current, a supply current of at least 500 mA is recommended. The total capacity of the external filter capacitor should be greater than 10 μF.
  • Place the filter capacitors C1 and C2 near the power pin of the module.

• Pins of the module

  • The reset pin or enable pin of the module is a hardware reset pin. The module has internal weak pull-up resistors configured. If the pin is not used, it can float. If a module has been paired, this pin cannot be used to clear pairing information.
  • When pin initialization is in progress after a system reboot, the user UART is in a high impedance state.
  • Other unused pins can float.
  • For more information about the pin definition, see the datasheet of each module.

• Power-on sequence of the module

  • Every time a module is powered on, the voltage ready time is not greater than 5 ms.

    

• Auto-baud detection firmware

  • To detect the baud rate of the MCU, the program will turn on and off the UART peripheral several times. When UART is turned off, its TX and RX pins will be in a high impedance state. If you plan to use a level translator circuit, make sure the signal lines remain the level in idle state when the TX and RX pins are high impedance.

• Antenna clearance description

  • Do not use metal shells or plastic shells with metallic paint or coating in the direction of the antenna radiation. Do not use metal objects such as screws and rivets near the antenna, which might affect the antenna efficiency.

  • Try to increase the distance from the top shell to the antenna to minimize the impact on antenna performance.

    

  • Try to increase the distance from the upper and bottom shells to the antenna to minimize the impact on antenna performance.

    

  • Keep the module away from speakers, power switches, cameras, HDMI, USB, and other high-speed signals to avoid interference.

  • Avoid metal shielding near the antenna. If co-channel interference occurs, you must evaluate the impact on the antenna performance and ensure the isolation from interference.

• Placement

  • Horizontal placement

    We recommend that you place the module at the edge of the backplane with the antenna facing outward, and flush the module’s GND terminal with the backplane’s GND terminal. Both terminals are fully connected.

    

  • Embedded placement

    Embed the module into the backplane through a slot that is flushed with or deeper than the module’s GND terminal. The side of the slot must be 15 mm or farther from the module’s board edge. A wider slot can achieve better performance that is still weaker than that of horizontal placement.

    

  • Vertical placement

    Insert the module vertically into the backplane slot with the antenna facing upward. The module’s GND terminal and the backplane’s GND terminal must be fully connected. We recommend that you keep a clearance distance of 15 mm or more around the antenna.

Low power design considerations

• Control the on/off condition of the module to achieve overall low power consumption.

  

  As shown in the circuit diagram, the MCU can control the switch S1 with the GPIO pin to power on/off the module. When the MCU has data to report to the cloud, it turns the S1 on. Then, the module can receive data from the MCU and report data to the cloud and the client. When data reporting is completed, the module will be powered off and consume no power.

  Disadvantage: Current sinking occurs. When the switch S1 is turned off, the current from the MCU can flow into the VCC pin of the module through the UART pin because the TXD and RXD pins of the MCU and the module are still connected. The TXD and RXD pins of the module are high so the current sinking will increase the consumption of the module. This might also cause the module to not operate correctly.

  Solution 1: Optimize the software of the MCU without hardware modification.

  When the MCU detects the data reporting is completed, the program proceeds with the following steps.

  1. Set the TXD and RXD pins of the MCU as GPIO pins that are configured as the open-drain or weak pull-down mode.
  2. Turn the switch S1 off to power off the module.
  3. This way, when the MCU has data to report, it turns the S1 on firstly.
  4. Then, it configures the TXD and RXD pins as the UART to establish communication with the module for data transmission.
  This solution does not apply to MCUs whose UART pin cannot be configured as the open-drain or weak pull-up mode. If the UART circuit has a pull-up resistor, one terminal of the resistor must be connected to the VCC pin of the module, or you can directly remove this resistor.

  Solution 2: Add a level translator to the circuit without software modification. See the circuit diagram in the preceding sections Level translator reference and Connection between a module and a 3.3V MCU and embed a level translator in the UART circuit.

• Pull down the module’s clock enable (CEN) pin or reset (RST) pin to reduce the idle consumption.

  

  As shown in the circuit diagram, the MCU can control the CEN or RST pin with the GPIO pin to power on/off the module. When the MCU has data to report to the cloud, the GPIO outputs high to power on the module. Then, the module can receive data from the MCU and report data to the cloud and the client. When data reporting is completed, the GPIO outputs low and the module runs in reset mode with low power consumption.

  Disadvantage: If the CEN or RST pin has an internal pull-up resistor, when the module runs in reset mode, this resistor can consume power.

RF test items and metrics

The antenna is susceptible to the distance from the shell to the surrounding components. We recommend that you test the radio frequency (RF) performance after the final test. The RF test items and metrics are listed in the following table.