HCS473 datasheet(34/68 Pages) MICROCHIP

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HCS473

DS40035C-page 32

Preliminary

 2002 Microchip Technology Inc.

FIGURE 3-20:

LF DATA RESPONSE ADJUSTMENTS (LFRSP=1)

FIGURE 3-21:

LF COMMUNICATION ANALOG DELAYS

3.2.7

RECEIVE STABILITY -

CALCULATING COMMUNICATION

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 (V

DD >

3.5V) and cold temperature. When the internal oscilla-

tor varies, both its transmitted T

E 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 V

DD 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 T

E. 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 T

E 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 T

E calculation method

would be to time an 8T

E sequence from the first Field

Acknowledge, then divide the value down to determine

the single T

E 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).

IFF, Read and Request Hop

CRC

RF Response (RFRSP=1)

Inductive

(LC Pins)

RF Response

(DATA Pin)

32-Bit LF Response (LFRSP=1)

TFINH

1 LFTE

Command

TCMD

Resulting 125 kHz on

Transponder Reader

Digital Representation

of Communication from

Transponder Reader

Antenna (TX)

600

µs

400

µs

Resulting 125 kHz on

Transponder Card

Antenna (RX)

Resulting digital signal

processed by HCS473,

after analog filter

600

µs

400

µs

TFILTR

TFILTF

TANTR

TANTF

Bit ‘1’, 200

µs LFTE

Bit ‘0’, 200

µs LFTE

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