HCS200_11 Datasheet, PDF (7/34 Pages) Microchip Technology – KEELOQ® Code Hopping Encoder 32-bit hopping code

checked against the receiver’s stored value to verify
that the decryption process was valid. If the discrimi-
nation value was programmed as the 12 LSb’s of the
serial number then it may merely be compared to the
respective bits of the received serial number; saving
EEPROM space.
3.6.2 BAUD RATE SELECT BIT (BSL0)
BSL0 selects the speed of transmission and the code
word blanking. Table 3-3 shows how the bit is used to
select the different baud rates and Section 5.2 provides
detailed explanation in code word blanking.
TABLE 3-3: BAUD RATE SELECT
BSL0
0
1
Basic Pulse
Element
400 μs
200 μs
Code Words
Transmitted
All
1 out of 2
3.6.3
LOW VOLTAGE TRIP POINT
SELECT (VLOW SEL)
The low voltage trip point select bit tells the HCS200
what VDD level is being used. This information will be
used by the device to determine when to send the volt-
age 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 to zero,
the VDD level is assumed to be 6.0 volts. Refer to
Figure 3-1 for voltage trip point.
FIGURE 3-1:
Volts (V)
5.5
5.0
4.5
4.0
3.5
3.0
2.5
VOLTAGE TRIP POINTS
BY CHARACTERIZATION
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)
HCS200
4.0 TRANSMITTED WORD
4.1 Code Word Format
The HCS200 code word is made up of several parts
(Figure 4-1). Each code word contains a 50% duty
cycle preamble, a header, 32 bits of encrypted data and
34 bits of fixed data followed by a guard period before
another code word can begin. Refer to Table 9-3 for
code word timing.
4.2 Code Word Organization
The HCS200 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 32 bits of Encrypted Data are generated from 4
button bits, 12 discrimination bits and the 16-bit sync
value. The encrypted portion alone provides up to four
billion changing code combinations.
The 34 bits of Fixed Code Data are made up of 1 sta-
tus bit, 1 fixed bit, 4 button bits and the 28-bit serial
number. The fixed and encrypted sections combined
increase the number of code combinations to 7.38 x
10¹⁹.
4.3 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 Pro-
gramming mode start-up sequence must be executed
as shown in Figure 4-3. If either S1 or S0 is set on the
falling edge of S2, 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 reserved bits are transmitted at the end of the
code word. The reserved bits can be ignored. The but-
ton code will be the S0, S1 value at the falling edge of
S2. The timing of the PWM data string is controlled by
supplying a clock on S2 and should not exceed 20 kHz.
When in Synchronous Transmission mode S2 should
not be toggled until all internal processing has been
completed as shown in Figure 4-3.
© 2011 Microchip Technology Inc.
DS40138D-page 7