RFHCS362AG datasheet(4/60 Pages) MICROCHIP

4 / 60 page

rfHCS362G/362F

DS41189A-page 4

Preliminary

- 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.

FIGURE 1-1:

ADDITIONAL BUTTON INPUTS

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.

VDD

RFEN

B4 B3 B2 B1 B0

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