HCS412_11 Datasheet, PDF (8/52 Pages) Microchip Technology – KEELOQ® Code Hopping Encoder and Transponder Burglar alarm systems

HCS412
2.2.7.2 TRANSPONDER INTERFACE
Connecting an LC resonant circuit between the LC0
and the LC1 pins creates the bi-directional low fre-
quency communication path with the HCS412.
The internal circuitry on the HCS412 provides the fol-
lowing functions:
• LF input amplifier and envelope detector to detect
and shape the incoming low frequency excitation
signal.
• 10V zener input protection from excessive
antenna voltage generated when proximate to
very strong magnetic fields.
• LF antenna clamping transistors for inductive
responses back to the transponder reader. The
antenna ends are shorted together, ‘clamped’,
dissipating the oscillatory energy. The reader
detects this as a momentary load on its excitation
antenna.
• Damping circuitry that improves communication
when using high-Q LC antenna circuits.
• Incoming LF energy rectification and regulation
for the supply voltage in battery-less or low bat-
tery transponder instances.
During normal transponder operation, the LC1 pin func-
tions to bias the LC0 AGC amplifier input. The amplifier
gain control sets the optimum level of amplification in
respect to the incoming signal strength. The signal then
passes through an envelope detector before interpreta-
tion in the logic circuit.
2.2.7.3 RF ENABLE OUTPUT
When the RF enable (RFEN) configuration option is
enabled, the RFEN signal output is coordinated with
the DATA output pin to provide typical ASK or FSK PLL
activation.
TABLE 2-1:
RFEN
0
1
RFEN OPTION
Description
RF Enable output is disabled.
RF Enable output is enabled.
TABLE 2-2: S2/RFEN/LC1 CONFIGURATION OPTION
S2LC
Resulting S2/RFEN/LC1 Configuration
0
• LC1 low frequency antenna output driver for inductive responses and LC bias.
Note: LC0 low frequency antenna input is also enabled.
• S2 button input pin with Schmitt Trigger detector and internal pull-down resistor.
1
• RFEN output driver.
Note: LC0 and LC1 low frequency antenna interfaces are disabled and the transponder circuitry is
switched off to reduce standby current.
3.0 ENCODER OPERATION
3.1 Encoder Activation
3.1.1 BUTTON ACTIVATION
The main way to enter Encoder mode is when the
wake-up circuit detects a button input activation; button
input transition from GND to VDD. The HCS412 control
logic wakes and delays a switch debounce time prior to
sampling the button inputs. The button input states,
cumulatively called the button status, determine
whether the HCS412 transmits a code hopping or seed
transmission, Table 3-1.
Additional button activations added during a transmis-
sion will immediately RESET the HCS412, perhaps
leaving the current code word incomplete. The device
will start a new transmission which includes the
updated button code value.
Buttons removed during a transmission will have no
effect unless no buttons remain activated. If no button
activations remain, the minimum number of compete
code words will be completed (Section 3.4.1) and the
device will return to Standby mode.
3.1.2 PROXIMITY ACTIVATION
The other way to enter Encoder mode is if the S2/LC
option is configured for LC operation and the wake-up
circuit detects a signal on the LC0 LF antenna input pin.
This form of activation is called Proximity activation as
a code hopping transmission would be initiated when
the device was proximate to a LF field.
Refer to Section 4.4 for details on configuring the
HCS412 for Proximity Activation.
DS41099D-page 8
© 2011 Microchip Technology Inc.