HCS512

© 1997 Microchip Technology Inc.

DS40151C-page 7

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

4.4

Synchronization with Decoder

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

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

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

?

Transmission

Received

Does

Ser # Check Val

Match

?

Decrypt Transmission

Is

Decryption

Valid

?

Is

Counter

Within 16

?

Is

Counter

Within 32K

?

Update

Counter

Execute

Command

Save Counter

in Temp Location

Start

No

No

No

No

Yes

Yes

Yes

Yes

Yes

No

and

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.

Blocked

Entire Window

rotates to eliminate

previously

used codes

Current

Position

(48K Codes)

Double

Operation

(16K

Single Operation

Window (16 Codes)

Codes)