RFHCS362G_11 Datasheet, PDF (3/54 Pages) Microchip Technology – KEELOQ® Code Hopping Encoder with UHF ASK/FSK Transmitter

- Normal Learning
The receiver uses information transmitted
during normal operation to derive the encryp-
tion key and decrypt the received code
word’s encrypted portion.
- Secure Learn
The transmitter is activated through a special
button combination to transmit a stored 60-bit
seed value used to generate the transmitter’s
encryption key. The receiver uses this seed
value to derive the same encryption key and
decrypt the received code word’s encrypted
portion.
• Manufacturer’s code – A unique and secret 64-
bit number used to generate unique encoder
encryption keys. Each encoder is programmed
with a encryption key that is a function of the man-
ufacturer’s code. Each decoder is programmed
with the manufacturer code itself.
1.2 Applications
The rfHCS362G/362F is suited for secure wireless
remote control applications. The EEPROM technology
makes customizing application programs (transmitter
codes, appliance settings, etc.) extremely fast and con-
venient. The small footprint packages are suitable for
applications with space limitations. Low-cost, low-
power, high performance, ease of use and I/O flexibility
make the rfHCS362G/362F very versatile. Typical
application circuits are shown in Figure 1-5 and
Figure 1-6.
Most low-end keyless entry transmitters are given a
fixed identification code that is transmitted every time a
button is pushed. The number of unique identification
codes in a low-end system is usually a relatively small
number. These shortcomings provide an opportunity
for a sophisticated thief to create a device that ‘grabs’
a transmission and retransmits it later, or a device that
quickly ‘scans’ all possible identification codes until the
correct one is found.
The rfHCS362G/362F, on the other hand, employs the
KEELOQ code hopping technology coupled with a trans-
mission length of 66 bits to virtually eliminate the use of
code ‘grabbing’ or code ‘scanning’. The high security
level of the rfHCS362G/362F is based on patented
technology. A block cipher based on a block length of
32 bits and a key length of 64 bits is used. The algo-
rithm obscures the information in such a way that even
if the transmission information (before coding) differs
by only one bit from that of the previous transmission,
the next coded transmission will be completely differ-
ent. Statistically, if only one bit in the 32-bit string of
information changes, approximately 50 percent of the
coded transmission bits will change.
rfHCS362G/362F
FIGURE 1-1: ADDITIONAL BUTTON INPUTS
VDD
B4 B3 B2 B1 B0
S0
S1
S2
RFEN
Up to 7 button inputs can be implemented making them
look like a binary value to the 3 Sx inputs. This is done
with switching diodes as shown in Figure 1-1. The dis-
advantage is that simultaneously pressed buttons now
appear as if a single button is pressed.
The rfHCS362G/362F has a small EEPROM array
which must be loaded with several parameters before
use. These are most often programmed by the manu-
facturer at the time of production. The most important
of these are:
• A 28-bit serial number, typically unique for every
encoder
• An encryption key
• An initial 16-bit synchronization value
• A 16-bit configuration value
The encryption key generation typically inputs the
transmitter serial number and 64-bit manufacturer’s
code into the key generation algorithm (Figure 1-2).
The manufacturer’s code is chosen by the system
manufacturer and must be carefully controlled as it is a
pivotal part of the overall system security.
The 16-bit synchronization counter is the basis behind
the transmitted code word changing for each transmis-
sion; it increments each time a button is pressed. Due
to the code hopping algorithm’s complexity, each incre-
ment of the synchronization value results in about 50%
of the bits changing in the transmitted code word.
© 2011 Microchip Technology Inc.
DS41189B-page 3