R
EM4223
Data decoding
The binary data ‘0’ and ‘1’ are extracted from the
incoming data stream by comparing the inter-pulse
interval with a pivot point. The pivot point has a value of
1.5Tari and is derived from the 3Tari interval contained in
the 2nd part of the SOF symbol. If the interval is less than
the pivot, then the binary value is ‘0’ and if it is greater
than the pivot then the binary value is ‘1’.
If the Transponder detects an invalid code it discards the
frame and waits for an unmodulated carrier of EOF
duration. No Error Messages are sent to the Reader.
Bits and byte ordering
Coding of data into symbols is MSB first. The coding for
the 8 bits of hex byte 'B1' is shown in Fig. 12.
1
0
1
1
0
0
0
1
t
0
Ts
Fig. 12 - Example of PIE byte encoding for 'B1'
Reader to Transponder 5 bit CRC (CRC-5)
The CRC-5 is used only for commands from the Reader
to the Transponder. All commands have a CRC-5 as the
last 5 bits of the first 16 bit part of an Extended command
or as the last 5 bits of a Short Command. The CRC-5 is
calculated on all the command bits after the SOF up to
the end of the Extended Parameters (11 bits in total –
see Fig. 3).
Reply to the Reader and during the 2 Transponder bit
periods following a Reply transmission.
In the case of the Next_Slot command the command is
interpreted by the Transponder in one of two ways.
If a Next_Slot command is received such that the
first pulse of the command is received during the
active command window of the Transponder, which
follows a transmission by the Transponder and this
Next_Slot command contains a signature parameter
that matches that sent by the Transponder in its last
transmission, then the command will be interpreted
as an instruction for that Transponder to move to the
quiet state
The polynomial used to calculate the CRC-5 is x^5 + x^3
+1. The CRC-5 register is pre-loaded with '01001' (MSB
(C4) to LSB (C0)) prior to commencing a CRC-5
calculation in both the case of a Reader to Transponder
command transmission and the case of a Transponder
initializing its CRC-5 register prior to receiving
command from the Reader.
a
Fig. 13 and below show the timing of the
Transponder command window.
The 5 bits of the CRC-5 embedded in the command are
received MSB first by the Transponder. The Transponder
will clock the first 16 bits of an Extended command or a
complete Short Command through its CRC-5 register as
it is receiving the command from the Reader and if these
16 bits were received without error, the Transponder’s
CRC-5 register will contain all zeros after the last bit has
been clocked through.
If a Next_Slot command is received at any time
other than coincident with an active command
window (opened by the Transponder following a
transmission) or if the Transponder had not
transmitted a Reply immediately prior to receiving
the NEXT_SLOT command or if the Next_Slot
command does not contain a signature parameter
that matches that sent by the Transponder in its last
transmission then the command is interpreted as an
instruction to step the current slot counter value in
Command Decoder
The Transponder can receive commands from a Reader
at any time other than the time that it is transmitting a
ISO mode or as
a
command to exit the
ROUND_STANDBY state in either ISO or FST
modes.
17
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