HCS473 Datasheet, PDF (32/68 Pages) Microchip Technology – Code Hopping Encoder and Transponder

HCS473
3.2.3.7 ANTI-COLLISION OFF
Anticollision is enabled/disabled for a given device by
the ACOL configuration option. The ANTICOLLISION
OFF command may be used to temporarily bypass
anticollision requirements for a single communication
sequence. It allows communication to an anticollision
enabled HCS473 if the VID and TID are not known
(perhaps during a learning sequence).
The command must immediately follow the wake-up
sequence, Figure 3-18. The HCS473 acknowledges
the command receipt, then reacts to all commands
even if the anticollision (ACOL) configuration option is
enabled and a SELECT TRANSPONDER sequence
has not been performed.
The next command must begin TCMD after the
Acknowledge.
The HCS473 remains in this anticollision off state until
the next time the device returns to SLEEP - communi-
cation error or TCMD without receiving another com-
mand. Multiple commands may therefore be sent
without sending the ANTICOLLISION OFF command
prior to each command.
FIGURE 3-18:
ANTICOLLISION OFF SEQUENCE (RF RESPONSE EXAMPLE)
‘111₂’ 2 bits
‘XXX’
WAKE ACK
TCMD
CMD
CRC
ACK
TCMD
CMD
Next
Command
Command CRC
Next
Command
TSF
Inductive In
(LC Pins)
RF Response
(DATA Pin)
TCMD
ACK
TCMD
TAOACK
TFINH
TCMD
5 LFTE
ACOL Off ACK
TCMD
3.2.4 LF RESPONSE CONSIDERATIONS
As LF responses are transmitted by the HCS473 plac-
ing a short across the LC antenna inputs, dissipating
the antenna resonance, the transponder reader must
still be sending the 125 kHz field for LF responses to
work. The low frequency field on-time (TFINH) must
therefore be approriately adjusted to receive an LF
Acknowledge sequence or LF data response, Figure 3-
19 and Figure 3-20.
3.2.5
CONSECUTIVE COMMAND
CONSIDERATIONS
Transponder commands may consecutively follow one
another to minimize communication time as the wake-
up sequence, device selection, anticollision off and
transport code presentation need not be repeated for
every command.
Consideration must be given to how long the transpon-
der reader keeps the LF signal on after the last data
bit’s rising edge (TFINH) when a command sequence...
• will be followed by another command sequence
• will result in a LF response
The reason is that the HCS473’s analog LF antenna
input circuitry will return to Low-power mode when the
125 kHz field remains absent; requiring a new wake-up
sequence to continue communication.
The HCS473’s analog section will never return to Low-
power mode during any TCMD window waiting for an LC
input communication edge, so long as the LF signal
existed up to the beginning of the TCMD window.
Please refer to Figure 3-19 and Figure 3-20 for exam-
ples on adjusting TFINH for consecutive commands and
LF responses.
3.2.6
LF COMMUNICATION ANALOG
DELAYS
LF communication edge delays result from the
HCS473’s internal analog circuit as well as the external
LC resonant antennas, Figure 3-21.
The rising and falling edge delays inherent to the
HCS473’s internal filtering are known and specified in
Table 7-5, TFILTR and TFILTF.
The cumulative rising and falling edge delays inherent
to both the series LC transmitting antenna and parallel
LC receiving antennas are design dependent, not a
HCS473 specification.
DS40035C-page 30
Preliminary
 2002 Microchip Technology Inc.