Integration of power:communication interfaces in smart true wireless headset designs
When designing such a system, it’s important to find a good trade-off between modulation current and modulation voltage level to ensure the system is not sensitive to external electromagnetic interference. On the other hand, the modulation current used also influences the overall power consumption of the communication system. Another tricky but important parameter beside the absolute modulation current is its slew rate. Steep current ramps may cause electro-magnetic emissions which can result in reception problems with mobile phones, Bluetooth, or FM radios. There are regulations that have to be fulfilled, otherwise a final product might not receive a license to be sold in some markets. Furthermore, the modulator is also the line reader which is intended to read the modulated data from the client device – indicated in blue in Figure 4 – whereas the green data represents data generated by the host to be send to the client.
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Figure 4: Example half duplex modulation protocol (Source: ams AG)
In this proposal, each frame is divided into 64 slots transferring 30 bits of data from the host to the client and vice versa. Each frame starts with a synchronization pulse that is generated by the host and which is necessary for a client to synchronize its clocks – as the host and the client don’t share the same clock, and so the client needs to extract its clock from the data stream and synchronization pulse. On the other end of the frame, the client terminates each frame with a synchronization pulse for the host to indicate both devices are in sync. Needless to say, this example requires some pre-synchronization sequencing which can be part of a possible host-detection circuit. This block on the client is necessary to ensure that data modulation only happens if both host and client are connected. For this purpose, a possible solution is that the host starts emitting pulses to explore if a client is connected to the power terminal. Once the Startup Sync Detector detects the synchronization pulses, it can wake up the MCU inside the earbud to start responding to the sync pulses, and to indicate presence of a valid client and so begin synchronizing with each other. Line reader and data modulator fulfil the same purpose as the host side – to read and transmit data to and from host.
The coil LCLIENT and RMODC are used to block high-frequency content and modulate data to the supply line. Furthermore, the resistors help to receive better signal integrity, but this is more relevant if longer signal lines are available in the system. For short signal traces there is no impedance matching of the transmission line and PCB necessary. A further important consideration when looking more closely at the transmission line is the DC resistance. To reduce charging time of a TWS earbud, it is important to keep DC resistance low to avoid big voltage drops which might lead to reduced charger current, due to low input voltage at the charger input. Small form factor coils in particular often provide a high resistance, which is contra-productive to our goal of keeping the resistance at a minimum level, and so maximize the charging current while minimizing the charging time for end users.
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Figure 5: Two-wire power and communication block diagram (Source: ams AG)
Certainly, the TWS system evolution is at the very beginning with all its features and form factors. However, it will quickly reach its peak due to a strong and rapid growing competitive market and demanding customers who are constantly trying to push the limits of physical design. Miniaturization combined with extended battery life is key to ensuring TWS systems can find their way in everyone’s life – perhaps without even being noticed. These key requirements lead to a general problem of the proposed implementation structure, shown in Figure 5. The system integration of the required blocks for the described communication interface (line reader, clock extraction unit, data modulator, startup sync detector) is of course not an easy task. Considering the existing size constraints inside small earbuds, it is most unlikely to be possible to use discrete components in the final form factor. In addition, the system inherits some complexity, therefore development requires good understanding and experience in analog and digital design. For many headphone companies, the effort is big enough to resign and stick with the drawbacks of adding additional poles, or simply not add any intelligence to their system.
An off-the-shelf power:communication (POW:COM) solution such as ams AG’s can significantly lower the hurdles of enabling a smart TWS system. This solution consists of the AS3442 (host device, inside the cradle) and the AS3447 (client device, inside the earbud) and adds even more functionality without adding too much more effort and development time. The interface to the AS3442/47 is a standard I2C interface to make integration effort low, whereas the communication between the two devices is a tailor-made communication interface which meets the technical requirements mentioned earlier. The interface provides a net data transfer rate of 1kBit/s. This data transfer rate includes all the necessary overhead as well as error handling to transfer error free data like battery status, serial numbers or user names. Data can be exchanged back-and-forth between cradle and earbud with simple I2C commands. Inside the AS3447, a dedicated memory space (“mailbox exchange register”) can be used to update e.g. the battery voltage level register. If the earbud MCU updates the value in the register, the cradle automatically gets an interrupt and is able to read out the value. That way, the cradle MCU always knows the battery voltage of the earbud and can decide if re-charging is necessary.
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Figure 6: POW:COM integration example True Wireless Earbud (Source: ams AG)
Of course, if the cradle battery becomes empty and needs to notify the earbud, the procedure is the same but in the other direction. In addition to the data exchange feature, the device offers several GPIOs which can be used to wake up or control external devices like the MCU, the Bluetooth SoC, external battery charger, sensors, or LEDs. A possible integration example of the POW:COM system is shown in Figure 6. It is quite clear to see that the complexity of the system shrinks tremendously if using the POW:COM system instead of multiple discrete functional blocks as shown previously. The integration of AS3442 and AS3447 into a TWS allows system designers to easily enable smart TWS systems while fulfilling the trend to miniaturization and extending battery life.
Horst Gether is Senior Product Manager and has joined ams AG in 2003 as application engineer for audio amplifiers and digital portable media players. In 2010 Horst became product manager for active noise cancelling products responsible for the technical product definition. He holds a master’s degree in automation engineering and is a member of the AAP Expert Listeners Panel.
Martin Denda is Staff Application Engineer and has joined ams AG in 2014 as application engineer for the business line audio & pressure sensors. His focus is on developing active noise cancellation headphones and customer support. Martin holds a master's degree in audio-engineering from the technical university of Graz.