Hardware Design Considerations

We provide a reference schematic for the CWTNU22 core module for use in secondary development. The key hardware design considerations are summarized below.

1. Power Supply Design

① Core module supply

-Recommended voltage: 3.3V (stable range 3.0V to 3.6V)

-Quiescent current: approximately below 20 mA

-Peak current: can reach 450 to 500 mA during Wi-Fi connection / data transfer (the power supply should provide at least 600 mA of margin to avoid disconnects or resets)

-Recommendation: use an LDO and place 10 uF + 0.1 uF decoupling capacitors close to the module.

② Audio playback section supply

-Supports 3.3V to 5V power supply

-When powered from 5V, add an M7 diode in series for voltage drop as shown in the core-board schematic to protect internal ICs

-Typical current: standby / system ringtone approximately 15 to 20 mA; with USB drive inserted approximately 40 to 60 mA

③ Power amplifier supply (user-designed)

-Amplifier ICs usually use 5V or higher voltage, for example the TPA31xx series commonly uses 5 to 24V

-Current demand is relatively high, from a few mA at mute to hundreds of mA or even several amperes at full power, so it should use an independent supply isolated from the core module to avoid noise coupling

-Recommendation: separate amplifier power from digital circuitry and use an inductor or LC filter to suppress high-frequency interference.

④ MCU section (user-designed)

A 3.3V MCU is recommended so that its UART voltage levels match the CWTNU22 core module directly, with no additional level shifter required.

The MCU mainly parses the core module’s UART protocol (115200 bps, 4 frames per second) and drives the display.

⑤ Other peripheral supplies (user-designed)

-Display panel (TFT / OLED / 7-segment, etc.): estimate current based on the selected panel and reserve 30% to 50% margin.

-High-power control (relays / external amplifiers, etc.): current variation may be large, so use an independent supply with sufficient margin to avoid voltage drop or overload.

-Overall recommendation: use a single input with multiple regulated rails and reserve adequate power margin according to the actual design to ensure stable operation.

Please refer to the schematic for the core module design and pay special attention to the required power-up logic levels.

UART1_TXID (pin 11) connects to the external MCU and must be held at a high level after power-up.

Most MCU GPIO pins are floating inputs during the early power-up stage and usually power up and initialize before the module. To avoid abnormal startup caused by the module sampling a low level by mistake, we strongly recommend:

-adding a 10k ohm pull-up resistor from the module-side UART1_TXID pin to 3.3V

-ensuring that the MCU firmware does not configure the corresponding pin as a low-level output during initialization

The music decoder section uses the KT404C chip.

The schematic shows the basic peripheral circuitry of the music chip, audio output, TF card socket, USB interface (supports USB drive playback), and the Flash memory circuit used to store system audio parameters.

Please refer to the download links at the bottom of this page for the full schematic and the KT404C datasheet.

During PCB layout, pay special attention to making the USB signal traces differential and length-matched.

This chip supports stereo output through DACL (left channel) and DACR (right channel).

The Flash model is 25Q128 and firmware must be programmed before mass production (firmware will be provided later).

Although it is compatible with most 25Q128 variants, to reduce risk we recommend prioritizing the vendor/model used during development verification. If the Flash manufacturer is changed, full verification must be completed before mass production.

To improve system stability, prevent the music chip from locking up abnormally, and effectively reduce standby power consumption, we recommend the power-control scheme shown above: power the music decoder circuit only when audio playback is required. We strongly recommend keeping this power-control section in the product design.

The example uses the 8002 audio power amplifier IC. Users may omit or redesign the amplifier section according to actual requirements.

The music chip outputs stereo signals. If a mono amplifier IC is used, it is recommended to mix the left and right channels through an RC network before feeding the amplifier input.

In the example, the amplifier on/off control is implemented using the core module’s SP_POWER pin together with the SP_BUSY signal from the music chip.

When the amplifier power is relatively large, it is recommended to use the core module’s PLAY_STATUS pin to control amplifier enable.

6. MCU + Display Section

This section’s hardware and firmware are designed by the user according to the actual product requirements, for example:

-Use a 7-segment LED display to show time, status, alarm count, and related information;

-Use a character or graphic LCD panel (LCD / OLED / TFT, etc.) for a richer UI;

-Or use other custom display approaches.

The MCU receives status, playback, time, and other data sent from the CWTNU22 core module over UART, parses the protocol packets, and drives the corresponding display hardware for real-time updates.

A 3.3V operating voltage is recommended for the MCU so that its IO levels fully match the CWTNU22 core module and avoid the added cost and potential issues of level-shifting circuitry.

We also provide a matching LCD display module solution, including detailed hardware schematics and sample reference firmware.

Customers can refer to the protocol parsing and handling logic in the sample program to shorten development time.

Note: the sample program is for reference only.

INFO Output Interface

The CWTNU22 core module transmits related information through the UART interface (UART1_TX). The UART parameters are currently fixed and cannot be changed.

This interface is mainly used to extend display functionality and can output time, alarm status, and related data. The system can still operate normally even if this interface is not used.

We provide two functional boards (the music alarm clock core board and the CWTNU22 evaluation board) plus one display module. Details are described below.

The design materials on this page are provided only for user reference and learning, with the goal of helping users get started quickly and carry out secondary development.

Reference design files: (see the Downloads tab)

-Schematic

-Complete project (LCSC EDA project)

If you need package libraries or the BOM, please export them from the project yourself.

The evaluation board follows the functional topology diagram in the solution overview to partition the hardware layout, making it easier to understand the role of each circuit section.

Main characteristics:

① Test socket design: The evaluation board uses a castellated pogo-pin socket so that the core module can be replaced quickly for functional verification.

② Multi-RTC support: Four RTC models are supported on the board and selected through jumpers, which is convenient for evaluating different RTC options.

③ Programming support: A built-in Type-C programming interface allows firmware to be flashed to the core module directly.

Notes:

① Programming jumpers: The auto-programming circuit uses four jumpers. Remove all of them after programming is complete so other functions are not affected.

② RTC selection jumpers: The RTC section has four pin headers corresponding to four RTC models (see the board silkscreen). Only one RTC option may be selected at a time.

Reference design files: (see the Downloads tab)

-Schematic

We provide a 1.77-inch TFT LCD display module (128 x 160 resolution, ST7735 driver) using the GD32E230F8P6TR MCU.

It can be used together with the CWTNU22 core board to display time, alarms, and other basic information.

Reference design files: (see the Downloads tab)

-Schematic

-LCD module reference code

1. Manufacturing Notes

1.1 Component procurement

-CWTNU22 core module:

The latest version is shipped by default (see the revision history for software changes). If you need an older specified version, please confirm with us in advance. The current software version can be checked on the [Info & Upgrade] page.

-KT404C music decoder chip:

When purchasing, be sure to confirm the chip’s check code with the supplier. The currently recommended / verified check code is 2CFE. Different batches or manufacturers may vary, and using a non-matching check code may cause abnormal music playback.

-25Q128 Flash chip:

We strongly recommend prioritizing the exact vendor and model used during development verification, such as common brands like Winbond or Macronix. If a different vendor must be used, complete thorough functional validation first, including firmware programming and music playback stability, before moving to volume procurement and production, in order to avoid compatibility risks.

1.2 Production preparation

-Programming the 25Q128:

Use a programmer to flash the audio firmware (see Downloads) into the Flash chip before SMT assembly.

The audio firmware size is close to 16M, so a programmer based on the CH347 chip is recommended. For large quantities, using a professional programming service is recommended.

2. Factory Test Procedure

2.1 Functions to be tested

After product assembly is complete, a factory functional test is required to ensure that all core modules are working properly. The main verification items are:

-Display function: after power-on, check whether the screen lights normally, whether the display is clear and complete, and whether there are obvious dead pixels or abnormal behavior.

-Networking: verify whether the Wi-Fi RF performance of the core module is normal. (Note: the module itself is 100% tested before shipment, but complete-product retesting is still required to guard against assembly or process issues.)

-RTC real-time clock (if the product includes this function): after power-off, verify whether timekeeping continues; after power-up, check whether time retention and reading are correct.

-Audio and storage related functions:

-Test whether the speaker / audio output can produce sound normally

-Verify that system ringtones and prompts can be played normally

-Check whether the TF card reader circuit works properly (recognized and able to play)

-Check whether the USB circuit works properly (USB drive recognized and able to play)

2.2 Preparation for factory test

① Wi-Fi (2.4G) test preparation:

-SSID: “iottimer”

-Password: “12345678”

A guest Wi-Fi hotspot on a router is recommended where possible, because phone hotspots usually support fewer clients and are less stable.

② TF card preparation:

Create a folder named “10” in the root directory of the TF card, and place audio files named in the form “001_xx” inside it (at least one file beginning with 001 is required).

③ USB drive preparation:

Create a folder named “10” in the root directory of the USB drive, and place audio files named in the form “001_xx” inside it (using content different from the TF card is recommended so playback can be distinguished easily).

2.3 Recommended factory test flow

① Insert the TF card and USB drive into their corresponding connectors.

② Hold the function key and power on the product:

-The device enters factory test mode

-The system voice prompt announces: “All parameters have been restored to factory defaults”

-All parameters are now reset to factory defaults.

③ Short press the function key again to enter “network test”:

-The device automatically connects to the “iottimer” hotspot and performs NTP network time synchronization

-After synchronization succeeds, the time is written to the RTC

-Both success and failure are reported through voice prompts

④ Short press the function key again to enter “TF card playback test”:

-The device automatically plays the “001_xx” audio in folder “10” on the TF card

⑤ Short press the function key again to enter “USB drive playback test”:

-The device automatically plays the “001_xx” audio in folder “10” on the USB drive

⑥ Power off, wait for a period of time, then power on again to verify whether timekeeping is normal:

Skip this step if the product does not use an RTC

-You can short press the key to hear the voice time announcement or check the current time on the display

-This is used to verify RTC backup retention and timekeeping accuracy

-Recommended power-off duration:

• To verify only whether time is retained: power off for at least 1 minute

• To verify external-crystal timekeeping accuracy: power off for at least 24 hours

Notes:

-The factory test flow supports interruption by power-off. For example, if only factory reset is needed, power can be removed after the voice prompt is heard.

-The status of each step in the factory test is output through the system information frame. The client can parse it and present the corresponding state or prompt on the display.

Detailed Explanation of Selected Settings

Most settings can be understood directly from the UI. A few special items are explained in detail below together with the interface.

Note: due to software upgrades and optimization, some page displays may differ slightly.

Upgrade Notes

Automatic push upgrades are not provided for now. If an upgrade is needed, please operate it manually from the web page: [Tools] -> [Device Upgrade]