HCS412 Datasheet, PDF (13/44 Pages) Microchip Technology – KEELOQ Code Hopping Encoder and Transponder

HCS412
FIGURE 3-5: PWM TRANSMISSION FORMAT—MOD = 0
TOTAL TRANSMISSION:
1 CODE WORD
Preamble Sync Encrypt Fixed
Guard Preamble Sync Encrypt
TE TE TE
LOGIC "0"
LOGIC "1"
31 RFTE Preamble, 50% Duty Cycle
Long Preamble (LPRE) disabled
10TE
Header
Encrypted
Portion
CODE WORD
Fixed Code
Portion
Guard
Time
FIGURE 3-6: MANCHESTER TRANSMISSION FORMAT—MOD = 1
TOTAL TRANSMISSION:
1 CODE WORD
Preamble Sync Encrypt Fixed
Guard Preamble Sync Encrypt
TE TE
LOGIC "0"
START bit
bit 0 _{bit 1} bit 2
LOGIC "1"
STOP bit
50% Duty Header
Preamble
Encrypted
Portion
CODE WORD
Fixed Code
Portion
Guard
Time
3.4 Encoder Special Features
3.4.1 CODE WORD COMPLETION AND
MINIMUM CODE WORDS
The code word completion feature ensures that entire
code words are transmitted, even if the active button is
released before the code word transmission is com-
plete. If the button is held down beyond the time for one
code word, multiple complete code words will result.
The device default is that a momentary button press
will transmit at least one complete code word. Enable
the Minimum Four Code Words (MTX4) configuration
option to extend this feature such that a minimum of 4
code words are completed on a momentary button acti-
vation.
3.4.2 AUTO-SHUTOFF
The Auto-shutoff function prevents battery drain should
a button get stuck for a long period of time. The time
period (TTO) is approximately 20 seconds, after which
the device will enter Time-out mode.
The device will stop transmitting in Time-out mode but
there will be leakage across the stuck button input’s
internal pull-down resistor. The current draw will there-
fore be higher than when in Standby mode.
3.4.3 CODE WORD BLANKING ENABLE
Federal Communications Commission (FCC) part 15
rules specify the limits on worst case average funda-
mental power and harmonics that can be transmitted in
a 100 ms window. For FCC approval purposes, it may
therefore be advantageous to minimize the transmis-
sion duty cycle. This can be achieved by minimizing the
on-time of the individual bits as well as by blanking out
consecutive code words.
© 2002 Microchip Technology Inc.
Preliminary
DS41099C-page 13