Linx Technologies CMD-KEY#-xxx - Manual (Page 2)

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THEORY OF OPERATION
The CMD-KEY#-*** Keyfob transmitter combines the LR Series transmitter with
an on-board Holtek HT640 encoder IC to form a simple, yet effective, RF remote
control transmitter. The LR transmitter is a low-cost, high-performance,
synthesized ASK / OOK transmitter. The transmitter’s synthesized architecture
delivers outstanding stability and frequency accuracy while minimizing the effects
of antenna port loading and mismatching. This reduces or eliminates frequency
pulling, bit contraction, and other negative effects common to SAW-based
transmitter architectures, providing a significantly higher level of performance
and reliability.
When a button is pressed on the transmitter, power is applied to the internal
circuitry and the encoder IC is enabled. The encoder detects the logic states of
the address lines and button data lines. These states are formatted into a three-
word transmission cycle that continues until the button is released. The encoder
data is used to modulate the transmitter, which, through the antenna, conveys
the data into free space. On the receiver side, a decoder IC or custom
microcontroller is used to check the transmitter’s address bits against the
address settings of the receiving device. If a match is confirmed, the decoder’s
outputs are set to replicate the transmitter’s button states. These outputs can
then be used to activate external circuitry required by the application.
The transmitter is compatible with several Linx receiver products, including the
LR, KH2, LT, and OEM product families. For applications where range is critical,
the LR Series receiver is the best choice due to its outstanding sensitivity. When
the transmitter is combined with an LR Series receiver and the HT658 decoder
chip, ranges of up to 750 feet are possible. Applications operating over shorter
distances will also benefit from the increased link reliability and superior noise
immunity provided by the LR Series receiver.
SETTING THE TRANSMITTER ADDRESS
The Keyfob allows the selection of one
of 1,024 unique addresses. All keyfobs
are supplied set to the same address.
To avoid contention with other units or
to create unique relationships, the
address can be changed. This is
accomplished by cutting the traces with
a sharp object, such as an X-Acto knife.
The traces are accessed by removing
the rear cover.
If the trace is intact, the address line is
connected to ground, otherwise it is
floating. The receiver’s address must match exactly in order for the units to
communicate. Application Note AN-00300 describes in detail how to set the
address to match any of the receivers offered by Linx. This note can be found in
the Support section of the Linx website, www.linxtechnologies.com.
BUTTON ASSIGNMENTS
The Keyfob is available in five unique button configurations. Those
configurations and the corresponding switch numbers are shown in the figure
below. The table shows which encoder data line has been assigned to each
switch. When a button is pressed, the data line will go high, causing the
corresponding data line on the decoder to go high if the addresses match.
S4
S2
S1 S3 S5
S5
S2
S5
S4
S4
S2
S4
S1 S3
S2
Figure 3: CMD-KEY#-*** Button Assignments
Figure 2: DIP Switch Assignments
Button Data Line
S1 D0
S2 D1
S3 D2
S4 D3
S5 D4
1. Characterized, but not tested
Notes
ELECTRICAL SPECIFICATIONS
Parameter Designation Min. Typical Max. Units Notes
POWER SUPPLY
Operating Voltage V
CC
2.1 3.0 3.6 VDC –
Supply Current I
CC
– 3.4 – mA –
Power-Down Current I PDN – 5.0 – nA 1
TRANSMITTER SECTION
Transmit Frequency Range: F
C
CMD-KEY#-315 – 315 – MHz –
CMD-KEY#-418 – 418 – MHz –
CMD-KEY#-433 – 433.92 – MHz –
Center Frequency Accuracy – -50 – +50 kHz –
ENVIRONMENTAL
Operating Temperature Range – -40 – +85 °C 1