HCS301 Datasheet, PDF (7/20 Pages) Microchip Technology – KEELOQ CODE HOPPING ENCODER

3.6.5 LOW VOLTAGE TRIP POINT SELECT
The low voltage trip point select bit is used to tell the
HCS301 what VDD level is being used. This information
will be used by the device to determine when to send
the voltage low signal to the receiver. When this bit is
set to a one, the VDD level is assumed to be operating
from a 9.0 volt or 12.0 volt VDD level. If the bit is set low,
then the VDD level is assumed to be 6.0 volts. Refer to
Figure 3-1 for voltage trip point.
The LED current limiting resistor value also affects the
LED current:
• VLOW = ^{1 (high limiting).}
• VLOW = ^{0 (low limiting).}
VLOW is tested at 3.5V and 13.0V.
FIGURE 3-1:
TYPICAL VOLTAGE TRIP
POINTS (BY
CHARACTERIZATION)
Volts (V)
5.5
5.0
4.5
4.0
3.5
3.0
2.5
VLOW sel = 0 ^VLOW
Max
Min
9.0
VLOW sel = 1
8.5
8.0
Max
7.5
7.0
Min
-40 -20 0 20 40 60 80 100
Temp (C)
HCS301
4.0 TRANSMITTED WORD
4.1 Transmission Format
The HCS301 transmission is made up of several parts
(Figure 4-1). Each transmission is begun with a
preamble and a header, followed by the encrypted and
then the fixed data. The actual data is 66 bits which
consists of 32 bits of encrypted data and 34 bits of fixed
data. Each transmission is followed by a guard period
before another transmission can begin. Refer to
Table 8-4 for transmission timing requirements. The
encrypted portion provides up to four billion changing
code combinations and includes the button status bits
(based on which buttons were activated) along with the
synchronization counter value and some discrimination
bits. The fixed portion is comprised of the status bits,
the function bits and the 28-bit serial number. The fixed
and encrypted sections combined increase the number
of combinations to 7.38 x 10¹⁹.
4.2 Synchronous Transmission Mode
Synchronous transmission mode can be used to clock
the code word out using an external clock.
To enter synchronous transmission mode, the program-
ming mode start-up sequence must be executed as
shown in Figure 4-3. If either S1 or S0 is set on the fall-
ing edge of S2 (or S3), the device enters synchronous
transmission mode. In this mode, it functions as a nor-
mal transmitter, with the exception that the timing of the
PWM data string is controlled externally and that 16
extra bits are transmitted at the end with he code word.
The button code will be the S0, S1 value at the falling
edge S2 or S3. The timing of the PWM data string is
controlled by supplying a clock on S2 or S3 and should
not exceed 20 KHz. The code word is the same as in
PWM mode with 16 reserved bits at the end of the
word. The reserved bits can be ignored. When in syn-
chronous transmission mode S2 or S3 should not be
toggled until all internal processing has been com-
pleted as shown in Figure 4-4.
4.3 Code Word Organization
The HCS301 transmits a 66-bit code word when a
button is pressed. The 66-bit word is constructed from
a Fixed Code portion and an Encrypted Code portion
(Figure 4-2).
The Encrypted Data is generated from 4 button bits, 2
overflow counter bits, 10 discrimination bits and the
16-bit sync value (Figure 8-5).
The Fixed Code Data is made up from two status bits,
four button bits and the 28-bit serial number. The four
button bits and the 28-bit serial number may be
encrypted with the Envelope Key if the envelope
encryption is enabled by the user.
© 1996 Microchip Technology Inc.
Preliminary
DS21143A-page 7