HCS473_13 Datasheet, PDF (18/68 Pages) Microchip Technology – KEELOQ® 3-Axis Transcoder

HCS473
FIGURE 3-8:
LED OPERATION
SWITCH Sx
DATA
Code Word
Code Word
Code Word
LED - VDD>VLOWSEL
(good battery)
LED - VDDVLOWSEL
(flat battery)
TLEDON
TLEDON
TLEDOFF
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
CRC1_{n + 1} = CRC0_n  Di_n
and
CRC0_{n + 1} = CRC0_n  Di_n  CRC1_n
with
CRC1 0₀ = 0
and Di_n 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.
Note: See Section 4.0, Programming Specs, for
information on programming OVR bits.
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.
DS40035D-page 18
Preliminary
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