Файл: wirelesspowerspecificationpart1.pdf

6.2.2Bit encoding scheme

The Power Receiver shall use a differential bi-phase encoding scheme to modulate data bits onto the Power Signal. For this purpose, the Power Receiver shall align each data bit to a full period tCLK of an internal clock signal, such that the start of a data bit coincides with the rising edge of the clock signal. This internal clock signal shall have a frequency kHz.

The Receiver shall encode a ONE bit using two transitions in the Power Signal, such that the first transition coincides with the rising edge of the clock signal, and the second transition coincides with the falling edge of the clock signal. The Receiver shall encode a ZERO bit using a single transition in the Power Signal, which coincides with the rising edge of the clock signal. Figure 6-2 shows an example.

tCLK

ONE ZERO ONE ZERO ONE ONE ZERO ZERO

Figure 6-2: Example of the differential bi-phase encoding

6.2.3Byte encoding scheme

The Power Receiver shall use an 11-bit asynchronous serial format to transmit a data byte. This format consists of a start bit, the 8 data bits of the byte, a parity bit, and a single stop bit. The start bit is a ZERO. The order of the data bits is lsb first. The parity bit is odd. This means that the Power Receiver shall set the parity bit to ONE if the data byte contains an even number of ONE bits. Otherwise, the Power Receiver shall set the parity bit to ZERO. The stop bit is a ONE. Figure 6-3 shows the data byte format—including the differential bi-phase encoding of each individual bit—using the value 0x35 as an example.

Figure 6-3: Example of the asynchronous serial format

6.2.4Packet structure

The Power Receiver shall communicate to the Power Transmitter using Packets. As shown in Figure 6-4, a Packet consists of 4 parts, namely a preamble, a header, a message, and a checksum. The preamble consists of a minimum of 11 and a maximum of 25 bits, all set to ONE, and encoded as defined in Section 6.2.2. The preamble enables the Power Transmitter to synchronize with the incoming data and accurately detect the start bit of the header.

The header, message, and checksum consist of a sequence of three or more bytes encoded as defined in Section 6.2.3.13

Figure 6-4: Packet format

The Power Transmitter shall consider a Packet as received correctly if:

The Power Transmitter has detected at least 4 preamble bits that are followed by a start bit.

The Power Transmitter has not detected a parity error in any of the bytes that comprise the Packet. This includes the header byte, the message bytes and the checksum byte.

The Power Transmitter has detected the stop bit of the checksum byte.

The Power Transmitter has determined that the checksum byte is consistent (see Section 6.2.4.3).

If the Power Transmitter does not receive a Packet correctly, the Power Transmitter shall discard the Packet, and not use any of the information contained therein. (Informative) In the ping phase as well as in the identification and configuration phase, this typically leads to a time-out, which causes the Power Transmitter to remove the Power Signal.

6.2.4.1Header

The header consists of a single byte that indicates the Packet type. In addition, the header implicitly provides the size of the message contained in the Packet. The number of bytes in a message is calculated from the value contained in the header of the Packet, as shown in the center column of Table 6-2.

Table 6-2: Message size

Table 6-3 lists the Packet types defined in this version 1.1.1 of the System Description Wireless Power Transfer, Volume I, Part 1. The formats of the messages contained in each of these Packet types are defined in Section 6.3. The format of the messages contained in Packet types, which are listed as Proprietary, is implementation dependent. Header values that are not listed in Table 6-3 are reserved. The Power Receiver shall not transmit Packets that have one of the reserved values as the header.

13The Power Receiver should turn off its communications modulator after transmitting a Packet. This may cause an additional HI state to LO state or LO state to HI state transition in the Primary Cell current.

*Header values not listed in this table correspond to reserved Packet types

6.2.4.2Message

The Power Receiver shall ensure that the message contained in the Packet is consistent with the Packet type indicated in the header. See Section 6.3 for a detailed definition of the possible messages. The first byte of the message, byte B0, directly follows the header.

6.2.4.3Checksum

The checksum consists of a single byte, which enables the Power Transmitter to check for transmission errors. The Power Transmitter shall calculate the checksum as follows:

,

where C represents the calculated checksum, H represents the header byte, and B0, B1,…, Blast represent the message bytes.

If the calculated checksum and the checksum byte contained in the Packet are not equal, the Power Transmitter shall determine that the checksum is inconsistent.

6.3Logical layer

This Section 6.3 defines the format of the messages of the communications interface.

6.3.1Signal Strength Packet (0x01)

Table 6-4 defines the format of the message contained in a Signal Strength Packet

Table 6-4: Signal Strength

Signal Strength Value The unsigned integer value in this field indicates the degree of coupling between the Primary Cell and Secondary Coil, with the purpose to enable Power Transmitters that use Free Positioning to determine the Primary Cell that provides optimum power transfer (see also Annex C). To determine the degree of coupling, the Power Receiver shall monitor the value of a suitable variable during a Digital Ping. Examples of such variables are:

The rectified voltage.

The open circuit voltage (as measured at the output disconnect switch).

The received Power (if the rectified voltage is actively or passively clamped during a Digital Ping).

The variable that is chosen shall result in a Signal Strength Value that increases monotonically with increasing degree of coupling. The Signal Strength Value is reported as

where is the monitored variable, and is the maximum value, which the Power Receiver expects for that variable during a Digital Ping. Note that the Power Receiver shall set the Signal Strength Value to 255 in the case that .