MCS3142-ISLASHST datasheet(18/37 Pages) MICROCHIP

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MCS3142

DS40001747A-page 18

 2014 Microchip Technology Inc.

5.0

ULTIMATE KEELOQ

TECHNOLOGY OPERATION

5.1

Synchronization Counter

The

synchronization

counter

always-incrementing,

event-based

counter.

The

counter is incremented whenever a new button

combination is registered and a new code word is

prepared.

For increased security, the synchronization counter will

not overflow. The device will cease operating when the

counter reaches its maximum value.

The initial value of the synchronization counter may be

set via the Synchronization Counter Initial Value

registers (see Table 3-21).

5.2

Encoder Time-Stamp

MCS3142 requires an external 31.768 kHz oscillator

connected to the secondary oscillator drive pins of the

internal timer. This timer is used to track the passage of

time over the lifetime of the encoder. Each Ultimate

KEELOQ

transmission

includes

this

time

with

quarter-second resolution (i.e., each count represents

one quarter of a second).

The initial value of the timer may be set at programming

time via the Timer Initial Value registers, described in

Table 3-24.

5.3

Function Code

The function code is a bitmapped representation of the

state of each button on the transmitter. States are

active-high.

5.4

Battery Level and Low Battery

Flag

Each Ultimate KEELOQ transmission contains a battery

level indicator byte. It includes a 7-bit digital

representation of the battery level and a 1-bit low

battery flag. The battery level is captured by measuring

an on-board 1.024V source using the battery as

reference. The low battery flag is high whenever the

measured battery voltage is estimated to be below

2.5V. Equation 5-1 converts the reference value into a

voltage.

EQUATION 5-1:

5.5

Button Press Timer

The button press timer is a high-resolution timer

representing the duration of the current button press at

the time the code word was prepared. Each count

represents 50 ms of time. It resets whenever a new

button combination is registered.

5.6

Delta Time

The delta time represents the elapsed time since the

previous code word was sent. The timer increments

every second.

5.7

Reset Counter

The Reset counter is an always-incrementing counter

representing the number of Power-on Reset events

experienced by the device. It is intended to be used by

the receiver as an indication that the transmitter has

been without power and that there will be a discrepancy

in the time-stamp.

For increased security, the Reset counter will not

overflow. The device will cease operating when the

counter reaches its maximum value.

The initial value of the Reset counter may be set using

the Reset Counter Configuration registers, described in

Table 3-23.

TABLE 5-1:

ULTIMATE KEELOQ®

FUNCTION CODE

TRANSLATION

Button

Function Code

xxx1

xx1x

x1xx

1xxx

VBAT

1.024 2



BATT

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