HCS360_01 Datasheet, PDF (16/28 Pages) Microchip Technology – KEELOQ® Code Hopping Encoder

HCS360
TABLE 6-1: PROGRAMMING/VERIFY TIMING REQUIREMENTS
VDD = 5.0V ± 10%
25° C ± 5 °C
Parameter
Symbol
Min.
Program mode setup time
T₂
0
Hold time 1
T₁
9.0
Program cycle time
TWC
—
Clock low time
TCLKL
25
Clock high time
TCLKH
25
Data setup time
TDS
0
Data hold time
TDH
18
Data out valid time
TDV
—
Max.
4.0
—
30
—
—
—
—
24
Units
ms
ms
ms
µs
µs
µs
µs
µs
7.0 INTEGRATING THE HCS360
INTO A SYSTEM
Use of the HCS360 in a system requires a compatible
decoder. This decoder is typically a microcontroller with
compatible firmware. Firmware routines that accept
transmissions from the HCS360 and decrypt the
hopping code portion of the data stream are available.
These routines provide system designers the means to
develop their own decoding system.
7.1 Learning a Transmitter to a Receiver
In order for a transmitter to be used with a decoder, the
transmitter must first be ‘learned’. Several learning
strategies can be followed in the decoder implementa-
tion. When a transmitter is learned to a decoder, it is
suggested that the decoder stores the serial number
and current synchronization value in EEPROM. The
decoder must keep track of these values for every
transmitter that is learned (Figure 7-1). The maximum
number of transmitters that can be learned is only a
function of how much EEPROM memory storage is
available. The decoder must also store the manufac-
turer’s code in order to learn a transmission transmitter,
although this value will not change in a typical system
so it is usually stored as part of the microcontroller
ROM code. Storing the manufacturer’s code as part of
the ROM code is also better for security reasons.
It must be stated that some learning strategies have
been patented and care must be taken not to infringe.
FIGURE 7-1: TYPICAL LEARN SEQUENCE
Enter Learn
Mode
Wait for Reception
of a Valid Code
Generate Key
from Serial Number
Use Generated Key
to Decrypt
Compare Discrimination
Value with Fixed Value
Equal
No
?
Yes
Wait for Reception
of Second Valid Code
Use Generated Key
to Decrypt
Compare Discrimination
Value with Fixed Value
Equal
No
?
Yes
Counters No
Sequential
?
Yes
Learn successful Store:
Serial number
Encryption key
Synchronization counter
Learn
Unsuccessful
Exit
DS40152D-page 16
 2001 Microchip Technology Inc.