HCS365 datasheet(11/40 Pages) MICROCHIP

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DS41109E-page 11

HCS365

3.2

Code Word Format

A KEELOQ code word consists of 32 bits of hopping

code data, 32 bits of fixed code data, and between 3 to

5 bits of status information. Various code word formats

are shown in Figure 3-1 and Figure 3-2.

3.2.1

HOPPING CODE PORTION

The hopping code portion is calculated by encrypting

the counter, discrimination value, and function code

with the Encoder Key (KEY). The hopping code is cal-

culated when a button press is debounced and remains

unchanged until the next button press.

The counter can be either a 16- or 20-bit counter. The

Configuration Option Counter Select (CNTSEL) value

will determine this. The counter select option must be

the same for both Encoder 1 and Encoder 2. If the 16-bit

counter is selected, the discrimination value is 10 bits

long and there are 2 counter overflow bits (OVR0,

OVR1). Set both bits in production and OVR0 will be

cleared on the first counter overflow and OVR1 on the

second.

If the counter is 20 bits, the discrimination value is 8 bits

long and there are no overflow bits. The rest of the 32

bits are made up of the function code also known as the

button inputs.

The discrimination value can be programmed with any

value to serve as a post decryption check on the

decoder end. In a typical system, this will be pro-

grammed with the 8 or 10 Least Significant bits of the

serial number. This will be stored by the receiver sys-

tem after a transmitter has been learned. The discrimi-

nation bits are part of the information that is to form the

encrypted portion of the transmission.

3.2.2

FIXED CODE PORTION

The 32 bits of fixed code consist of 28 bits of the serial

number (SER) and a copy of the 4-bit function code.

This can be changed to contain the whole 32-bit serial

number by setting the Extended Serial Number (XSER)

configuration option to a 1. If more than one button is

pressed, the function codes are logically OR’ed

together. The function code is repeated in the

encrypted and unencrypted data of a transmission.

TABLE 3-4:

FUNCTION CODES

3.2.3

STATUS INFORMATION

The status bits will always contain the output of the Low

Voltage (VLOW) detector and Cyclic Redundancy

Check (CRC). If Queue (QUEN) is enabled, button

queue information will be included in the code words.

FIGURE 3-1:

CODE WORD DATA FORMAT (16-BIT COUNTER)

Button

Function Code

xx1x

x1xx

1xxx

xxx1

Fixed Code Portion (32 Bits)

CRC

2 Bits

VLOW

1-Bit

SERIAL NUMBER

(28 Bits)

BUT

4 Bits

Counter

Overflow

2 Bits

DISC

10 Bits

Synchronization

16 Bits

Counter

OVR1

OVR0

Transmission Direction LSB First

Hopping Code Portion (32 Bits)

With XSER=0, 16-bit Counter, QUEN=0

Status Information

(3 Bits)

BUT

4 Bits

Fixed Code Portion (32 Bits)

QUE

2 Bits

CRC

2 Bits

VLOW

1-Bit

SERIAL NUMBER

(32 Bits)

Q1 Q0 C1 C0

BUT

4 Bits

Counter

Overflow

2 Bits

DISC

10 Bits

Synchronization

16 Bits

Counter

S0 S3

OVR1

OVR0

Hopping Code Portion (32 Bits)

With XSER=1, 16-bit Counter, QUEN=1

Status Information

(5 Bits)

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