HCS412 Datasheet, PDF (12/44 Pages) Microchip Technology – KEELOQ Code Hopping Encoder and Transponder

HCS412
FIGURE 3-4: SEED CODE WORD DATA FORMAT
QUE
2 Bits
Q1 Q0
CRC
2 Bits
C1 C0
VLOW
1-Bit
BUT
4 Bits
SDVAL3
12 Most Sig Bits
MSb
S2 S1 S0 LC0
SDVAL2
16 Bits
SDVAL1
16 Bits
Shaded data included in CRC calculation
Note: SEED transmissions only allowed when appropriate configuration bits are set.
SDVAL0
16 Least Sig Bits
LSb
69 Data bits
Transmitted
LSb first.
TABLE 3-4: SEED TRANSMISSION OPTIONS
SEED
TMPSD
Description
0
0
SEED transmissions permanently disabled
0
1
Limited SEED transmissions (Note 1) - temporarily enabled until the
7 LSb’s of the synchronization counter wrap from 7Fh to 00h
1
0
SEED transmissions permanently enabled (Note 1)
1
1
SEED transmissions permanently disabled (2 key IFF enabled)
Note 1: Refer to Table 3-1 for appropriate button activation of SEED transmissions.
3.3 Transmission Data Modulation
The data modulation format is selectable between
Pulse Width Modulation (PWM) and Manchester using
the Data Modulation (MOD) configuration option.
Regardless of the modulation format, each code word
contains a leading 50% duty cycle preamble and a syn-
chronization header to wake the receiver and provide
synchronization events for the receive routine. Each
code word also contains a trailing guard time, separat-
ing code words. Manchester encoding further includes
a leading and closing ‘1’ around each 69-bit data block.
The same code word repeats as long as the same input
pins remain active, until a time-out occurs or a delayed
seed transmission is activated.
The modulated data timing is typically referred to in
multiples of a Basic Timing Element (RFTE). ‘RF’ TE
because the DATA pin output is typically sent through a
RF transmitter to the decoder or transponder reader.
RFTE may be selected using the Transmission Baud
Rate (RFBSL) configuration option (Table 3-6).
TABLE 3-5: TRANSMISSION
MODULATION TIMING
Period
PWM Manchester Units
Preamble
31*
Header
10
Data
207
Guard
46
31*
RFTE
4
RFTE
142
RFTE
31
RFTE
* Enabling long preambles extends the first code
word’s preamble to TLPRE milliseconds.
TABLE 3-6: BAUD RATE SELECTION (RFBSL)
RFBSL1:0 CWBE
PWM RFTE
Manchester RFTE
Transmit...
00b
X
01b
0
1
10b
0
1
11b
0
1
400 µs
200 µs
200 µs
100 µs
100 µs
100 µs
100 µs
800 µs
400 µs
400 µs
200 µs
200 µs
200 µs
200 µs
All code words
All code words
Every other code word
All code words
Every other code word
All code word
Every fourth code word
DS41099C-page 12
Preliminary
© 2002 Microchip Technology Inc.