ST STEVAL-WLC38RX - FSK Communication; Figure 39. ASK Modulation Capacitor Connections; Figure 40. Example of Differential Bi-Phase Encoding

Figure 39. ASK modulation capacitor connections
5.11.1.1 ASK communication example: sending a proprietary packet from STWLC38 to STWBC86
The explanation of ASK communication is described in detail in Bi-directional communication application note on
STWLC38 and STWBC86.
5.11.2 FSK communication
The power transmitter modulates the power signal by switching between its normal operating frequency f
op
and its
modulated frequency f
mod
. The difference between f
op
and f
mod
can be described by two parameters: polarity
and depth.
Polarity is given by the difference between f
mod
and f
op
– a positive polarity corresponds to a positive difference,
while a negative polarity corresponds to a negative difference.
Depth describes the magnitude of difference between f
op
and f
mod
. Please note that a negative polarity results in
a higher induced voltage on the power receiver coil and should therefore be used with care.
The FSK communication is executed via packets. Each packet consists of 3 parts – a header, a message and a
checksum. The header is a single byte which indicates the packet type, while also implicitly providing the size of
the message part. The message size can range from 1 up to 27 bytes. The checksum is a single byte which
provides a way to check for transmission errors. It is calculated as an exclusive-OR of the header bytes and the
message bytes.
Each FSK bit is aligned to 512 periods of the power signal. A ONE bit is represented by two transitions over the
512-period slot - the frequency change occurs at the very start of the slot and after 256 power signal periods
pass. A ZERO bit is represented by a single transition (at the start of the 512-period slot).
Figure 40. Example of differential bi-phase encoding
UM3154
Bidirectional communication
UM3154 - Rev 2
page 32/81