HCS200

DS40138A-page 6

Preliminary

© 1996 Microchip Technology Inc.

3.6.2

BAUD RATE SELECT BITS (BSL0)

BSL0 selects the speed of transmission and the code

word blanking. Table 3-3 shows how the bits are used

to select the different baud rates and Section 5.2 pro-

vides detailed explanation in code word blanking.

TABLE 3-3:

BAUD RATE SELECT

3.6.3

LOW VOLTAGE TRIP POINT SELECT

The low voltage trip point select bit is used to tell the

will be used by the device to determine when to send

the voltage low signal to the receiver. When this bit is

Figure 3-1 for voltage trip point.

FIGURE 3-1:

VOLTAGE TRIP POINTS BY

CHARACTERIZATION

BSL0

Basic Pulse

Element

Code Words

Transmitted

0

400

µs

All

1

200

µs

1 out of 2

-40

20

40

100

8.5

7.5

8.0

7.0

9.0

Temp (C)

Volts (V)

-20

0

60

80

2.5

3.0

3.5

4.0

4.5

5.0

5.5

Max

Min

Max

Min

4.0

TRANSMITTED WORD

4.1

Transmission Format

The HCS200 transmission is made up of several parts

(Figure 4-1). Each transmission begins 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 por-

tion provides up to four billion changing code combina-

tions and includes the button status bits (based on

which buttons were activated) along with the synchroni-

zation 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

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, the device enters synchronous trans-

mission mode. In this mode, it functions as a normal

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. The timing of the PWM data string is con-

trolled by supplying a clock on S2 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 synchronous

transmission mode S2 should not be toggled until all

internal processing has been completed as shown in

Figure 4-3.

4.3

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 Encrypted Data is generated from 3 button bits,

12 discrimination bits, and the 16-bit sync value

(Figure 4-2).

The Fixed Code Data is made up from 1 status bit, 1

fixed bit, 4 button bits, and the 28-bit serial number.