HCS512 Datasheet, PDF (7/20 Pages) Microchip Technology – KEELOQ CODE HOPPING DECODER

4.3 Validation Steps
Validation consists of the following steps:
• Search EEPROM to find the Serial Number Check
Value Match
• Decrypt the Hopping Code
• Compare the 10 bits of discrimination value with
the lower 10 bits of serial number
• Check if the synchronization counter falls within
the first synchronization window.
• Check if the synchronization counter falls within
the second synchronization window.
• If a valid transmission is found, update the syn-
chronization counter, else use the next transmitter
block and repeat the tests.
FIGURE 4-2: DECODER OPERATION
Start
No Transmission
Received
?
Yes
No
Does
Ser # Check Val
Match
?
Yes
Decrypt Transmission
Is
No _Decryption
Valid
?
Yes
Is
Counter
Yes
Within 16
?
No
Is
No Counter
Within 32K
?
Yes
Save Counter
in Temp Location
Execute
Command
and
Update
Counter
4.4 Synchronization with Decoder
The KEELOQ technology features a sophisticated
synchronization technique (Figure 4-3) which does not
require the calculation and storage of future codes. If
the stored counter value for that particular transmitter
and the counter value that was just decrypted are within
a formatted window of 16, the counter is stored and the
HCS512
command is executed. If the counter value was not
within the single operation window, but is within the
double operation window of 16K, the transmitted syn-
chronization value is stored in a temporary location,
and it goes back to waiting for another transmission.
When the next valid transmission is received, it will
check the new value with the one in temporary storage.
If the two values are sequential, it is assumed that the
counter was outside of the single operation ‘window’,
but is now back in sync, so the new synchronization
value is stored and the command executed. If a trans-
mitter has somehow gotten out of the double operation
window, the transmitter will not work and must be
relearned. Since the entire window rotates after each
valid transmission, codes that have been used become
part of the ‘blocked’ (48K) codes and are no longer
valid. This eliminates the possibility of grabbing a previ-
ous code and retransmitting it to gain entry.
FIGURE 4-3: SYNCHRONIZATION WINDOW
Entire Window
rotates to eliminate
previously
used codes
Blocked
(48K Codes)
Current
Position
Double
Operation
(16K
Codes)Single _Operation
Window (16 Codes)
4.5 Sleep Mode
The sleep mode of the HCS512 is used to reduce cur-
rent consumption when no RF input signal is present.
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 significantly
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)
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.
© 1997 Microchip Technology Inc.
DS40151C-page 7