HCS473 Datasheet, PDF (34/68 Pages) Microchip Technology – Code Hopping Encoder and Transponder

HCS473
FIGURE 3-20:
LF DATA RESPONSE ADJUSTMENTS (LFRSP=1)
IFF, Read and Request Hop
CRC
32-Bit LF Response (LFRSP=1)
Next
Command
Inductive
(LC Pins)
RF Response
(DATA Pin)
RF Response (RFRSP=1)
1 LFTE
TFINH
TCMD
TCMD
FIGURE 3-21:
LF COMMUNICATION ANALOG DELAYS
Bit ‘1’, 200 µs LFTE
600 µs
Digital Representation
of Communication from
Transponder Reader
Resulting 125 kHz on
Transponder Reader
Antenna (TX)
Bit ‘0’, 200 µs LFTE
400 µs
Resulting 125 kHz on
Transponder Card
Antenna (RX)
Resulting digital signal
processed by HCS473,
after analog filter
TANTR
TFILTR
TFILTF
600 µs
TANTF
400 µs
3.2.7
RECEIVE STABILITY -
CALCULATING COMMUNICATION
TE
The HCS473’s internal oscillator may vary ±10% over
the device’s rated voltage and temperature range for
commercial temperature devices. A certain percentage
of industrial temperature devices vary further on the
slow side, -20%, when used at higher voltages (VDD >
3.5V) and cold temperature. When the internal oscilla-
tor varies, both its transmitted TE and expected TE
when receiving will vary.
The HCS473 receive capability is ensured over a ±10%
oscillator variance, with receive capability no longer
robust as oscillator variance approaches ±15%. Indus-
trial devices operating at VDD voltages greater than
3.5V (and cold temperature) are therefore not guaran-
teed to be able to properly receive when communicated
to using an exact TE. When designing for these specific
operating conditions, the system designer must imple-
ment a method to adjust communication timing to the
speed of the HCS473.
Communication reliability with the transponder may be
improved by the transponder reader calculating the
HCS473’s TE from the Field Acknowledge sequence
and using this exact time element in communication to
and in reception routines from the transponder.
Always begin and end the time measurement on rising
edges. Whether LF or RF, the falling edge decay rates
may vary but the rising edge relationships should
remain consistent. A common TE calculation method
would be to time an 8TE sequence from the first Field
Acknowledge, then divide the value down to determine
the single TE value. An 8 TE measurement will give
good resolution and may be easily right-shifted (divide
by 2) three times for the math portion of the calculation
(Figure 3-22).
DS40035C-page 32
Preliminary
 2002 Microchip Technology Inc.