HCS473-SLROM datasheet(20/68 Pages) MICROCHIP

20 / 68 page

HCS473

DS40035C-page 18

Preliminary

 2002 Microchip Technology Inc.

FIGURE 3-8:

LED OPERATION

3.1.6

CYCLE REDUNDANCY CHECK

(CRC)

The decoder can use the CRC bits to check the data

integrity before processing begins. The CRC is calcu-

lated on the previously transmitted bits (Figure 3-2),

detecting all single bit and 66% of all double bit errors.

EQUATION 3-1:

CRC CALCULATION

and

with

and Din the nth transmission bit 0 ≤ n ≤ 64

3.1.7

COUNTER OVERFLOW BITS

(OVR1, OVR0)

The Counter Overflow Bits may be utilized to increase

the 16-bit synchronization counter range from the nom-

inal 65,535 to 131,070 or 196,605. The bits do not exist

when the device is configured for 20-bit counter opera-

tion.

The bits must be programmed during production as ‘1’s

to be utilized. OVR0 is cleared the first time the syn-

chronization counter wraps from FFFFh to 0000h.

OVR1 is cleared the second time the synchronization

counter wraps to zero. The two bits remain at ‘0’ after

all subsequent counter wraps.

3.1.8

DISCRIMINATION VALUE (DISC)

The Discrimination Value is typically used by the

decoder in a post-decryption check. It may be any

value, but in a typical system it will be programmed

equal to the Least Significant bits of the serial number.

The discrimination bits are part of the information that

form the encrypted portion of the transmission

(Figure 3-2). After the receiver has decrypted a trans-

mission, the discrimination bits are checked against the

receiver’s stored value to verify that the decryption pro-

cess was valid; appropriate decryption key was used. If

the discrimination value was programmed as the LSb’s

of the serial number then it may merely be compared to

the respective bits of the received serial number.

The discrimination bit field size varies with the counter

select (CNTSEL) option (Figure 3-2).

3.2

Transponder Mode

The HCS473’s Transponder mode allows it to function

as a bi-directional communication transponder. Com-

mands are received on the LC pins, responses may be

returned on either the LC pins or DATA pin for short

range LF or long range RF responses, respectively.

Transponder mode capabilities include:

• A bi-directional challenge and response sequence

for IFF validation.

• Read selected EEPROM areas.

• Write selected EEPROM areas.

• Request a code hopping transmission.

• Proximity Activation of a code hopping transmis-

sion.

• Address an individual transponder when multiple

units are within the LF field; device selection for

anticollision communication purposes.

SWITCH Sx

LED -

DATA

LEDON

LEDOFF

Code Word

LEDON

DD>VLOWSEL

(good battery)

LED - V

≤VLOWSEL

(flat battery)

Note:

See Section 4.0, Programming Specs, for

information on programming OVR bits.

CRC 1

[]n1

CRC 0

[]n Din

⊕

CRC 0

[]n1

CRC 0

[]n Din

⊕

() CRC 1

[]n

⊕

CRC 1 0

[]0

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