HCS512_11 Datasheet, PDF (8/34 Pages) Microchip Technology – KEELOQ® Code Hopping Decoder system and high security

HCS512
4.4 Synchronization with Decoder
(Evaluating the Counter)
The KEELOQ technology patent scope includes a
sophisticated synchronization technique that does not
require the calculation and storage of future codes. The
technique securely blocks invalid transmissions while
providing transparent resynchronization to transmitters
inadvertently activated away from the receiver.
Figure 4-3 shows a 3-partition, rotating synchronization
window. The size of each window is optional but the
technique is fundamental. Each time a transmission is
authenticated, the intended function is executed and
the transmission's synchronization counter value is
stored in EEPROM. From the currently stored counter
value there is an initial "Single Operation" forward win-
dow of 16 codes. If the difference between a received
synchronization counter and the last stored counter is
within 16, the intended function will be executed on the
single button press and the new synchronization coun-
ter will be stored. Storing the new synchronization
counter value effectively rotates the entire synchroniza-
tion window.
A "Double Operation" (resynchronization) window fur-
ther exists from the Single Operation window up to 32K
codes forward of the currently stored counter value. It
is referred to as "Double Operation" because a trans-
mission with synchronization counter value in this win-
dow will require an additional, sequential counter
transmission prior to executing the intended function.
Upon receiving the sequential transmission the
decoder executes the intended function and stores the
synchronization counter value. This resynchronization
occurs transparently to the user as it is human nature
to press the button a second time if the first was unsuc-
cessful.
The third window is a "Blocked Window" ranging from
the double operation window to the currently stored
synchronization counter value. Any transmission with
synchronization counter value within this window will
be ignored. This window excludes previously used,
perhaps code-grabbed transmissions from accessing
the system.
Note:
The synchronization method described in
this section is only a typical implementation
and because it is usually implemented in
firmware, it can be altered to fit the needs
of a particular system.
FIGURE 4-3:
SYNCHRONIZATION WINDOW
Entire Window
rotates to eliminate
use of previously
used codes
Blocked
Window
(32K Codes)
Double Operation
(resynchronization)
Window
(32K Codes)
Stored
Synchronization
Counter Value
Single Operation
Window
(16 Codes)
4.5 SLEEP Mode
The SLEEP mode of the HCS512 is used to reduce
current consumption when no RF input signal is pres-
ent. SLEEP mode will only be effective in systems
where the RF receiver is relatively quiet when no signal
is present. During SLEEP, the clock stops, thereby sig-
nificantly reducing the operating current. SLEEP mode
is enabled by the SLEEP bit in the configuration byte.
The HCS512 will enter SLEEP mode when:
• The RF line is low
• After a function output is switched off
• Learn mode is terminated (time-out reached)
DS40151E-page 8
The device will not enter SLEEP mode when:
• A function output is active
• Learn sequence active
• Device is in Programming mode
The device will wake-up from SLEEP when:
• The SLEEP input pin changes state
• The CLOCK line changes state
Note:
During SLEEP mode the CLK line will
change from an output line to an input line
that can be used to wake-up the device.
Connect CLK to LRNIN via a 100K resistor
to reliably enter the Learn mode whenever
SLEEP mode is active.
© 2011 Microchip Technology Inc.