Figure 37. Example of a differential bi-phase encoding scheme
To transmit a single byte of data, the power receiver must send an 11-bit sequence. This sequence consists of a
START bit (ZERO), the 8 data bits of the byte (LSB first), a parity bit and a STOP bit (ONE). The parity is odd,
meaning an even number of ONE bits in the data byte results in the parity bit being equal to ONE, while an odd
number of ONE bits in the data byte results in the parity bit being equal to ZERO. An example of such message
can be seen in the picture below:
Figure 38. Example of asynchronous serial format
A packet consists of a preamble, header, message, and checksum. The preamble contains 11 to 25 ONE bits
and enables the power transmitter to synchronize with the incoming data. The header, message and checksum
are sequence of three or more bytes. The header indicates the packet type while also implicitly providing the size
of the message. The checksum consists of a single byte. It is calculated as an exclusive-OR of both the header
and message bytes and enables the power transmitter to check for transmission errors.
For a more detailed explanation please refer to the Qi Specification, Section Power Receiver to Power Transmitter
communications interface.
STWLC38 uses integrated low resistance N-channel MOSFET transistors to modulate the power signal. The
internal switches are connected to the COMA1, COMB1, COMA2 and COMB2 pins. As shown below, these pins
are connected to the AC1 and AC2 nodes through external capacitors, therefore connecting those capacitors
modulates the impedance seen by the coil, which is reflected onto the primary (transmitter) coil as a change in
current and/or voltage.
The figure below shows the ASK modulation capacitors connection, as well as optional ASK demodulation circuit,
which is required for a transmitter mode operation.
UM3154
Bidirectional communication
UM3154 - Rev 2
page 31/81
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