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ENCODER OPERATION
POWER SUPPLY REQUIREMENTS
The KH2 Series transmitter internally utilizes
the HT640 encoder from Holtek. The
Power On
The module does not have an internal voltage
regulator; therefore it requires a clean, well-regulated
Vcc TO
MODULE
encoder begins a three-word transmission
cycle when the Transmission Enable line
(TE) is pulled high. This cycle will repeat
itself for as long as the TE line is held high.
Standby Mode
power source. While it is preferable to power the unit
from a battery, it can also be operated from a power
supply as long as noise is less than 20mV. Power
supply noise can affect the transmitter modulation;
Vcc IN
10 Ω
10 μ F
Once TE falls low, the encoder output
therefore, providing a clean power supply for the
A 10 Ω resistor in series with the supply followed by a
completes its final cycle and then stops as
shown in the Encoder / Decoder Timing
diagram. When a transmission enable signal
is applied, the encoder scans and transmits
the status of the 10 bits of the address code
and the 8 bits of the data serially in the order
A0 to A9, D0 to D7.
The status of each address / data pin can be
individually preset to logic high, low, or
floating. The floating state on the data input
is interpreted as logic low by the decoders
since the decoder output only has two
states. The address pins are usually set to
No
Transmission
Enabled?
Yes
3 Data Words
Transmitted
Transmission
Still Enabled?
3 Data Words
Transmitted
Continuously
module should be a high priority during design.
Figure 10: Supply Filter
10μF tantalum capacitor from V CC to ground will help in cases where the quality
of supply power is poor. These values may need to be adjusted depending on
the noise present on the supply line.
DATA INPUTS
When the Transmit Enable (TE) line goes high, the states of the eight data input
lines are recorded and encoded for transmission. The data lines are tri-state,
which means that they can be high, low, or floating, though the decoder will
interpret the floating state as a low. This feature means that the data lines do not
require pull-up or pull-down resistors. The states of the data lines can be set by
switches, jumpers, microcontrollers, or hardwired on the PCB.
switches or PCB wiring, while the data is
transmit particular security codes by DIP
Figure 8: Encoder Flowchart
selected using push buttons or electronic
switches. The floating state allows the KH2 transmitter to be used without pull-
up or pull-down resistors on the data and address input lines.
Encoder
Transmit
The encoder will send the states of the address and data lines three times. If the
TE line is still high, it will begin the cycle again. This means that the states of the
data lines are refreshed with each cycle, so the data lines can be changed
without having to pull TE low. There can be up to a 150mS lag in response as
the transmitter finishes one cycle then refreshes and starts over.
ENABLING TRANSMISSION
Enable
Encoder
< 1 Word
The module’s Transmit Enable (TE) line controls transmission status. When
taken high, the module initiates transmission, which continues until the line is
2
Data Out
Decoder VT
3 Words
2 Words
14
Transmitted Continuously
Clocks
3 Words
2
14
Clocks
pulled low or power to the module is removed. In some cases this line will be
wired permanently to V CC and transmission controlled by switching V CC to the
module. This is particularly useful in applications where the module powers up
and sends a transmission only when a button is pressed on the remote.
Decoder
Data Out
Check
Check
1/2 Clock Time
1/2 Clock Time
USING LADJ
The LADJ line allows the transmitter’s output power to be easily adjusted for
range control, lower power consumption, or to meet legal requirements. This is
Figure 9: Encoder / Decoder Timing Diagram
SETTING THE TRANSMITTER ADDRESS
The module provides ten tri-state address lines. This allows for the formation of
up to 59,049 (3 10 ) unique transmitter-receiver relationships. Tri-state means that
the address lines have three distinct states: high, low, or floating. These pins
may be hardwired or configured via a microprocessor, DIP switch, or jumpers.
The receiver’s address line states must match the transmitter’s exactly for a
transmission to be recognized. If the transmitted address does not match the
receiver’s local address, then the receiver will take no action.
Page 6
done by placing a resistor between GND and LADJ. When LADJ is connected
directly to GND, the output power will be at its maximum. Placing a resistor will
lower the output power by up to 7dB, as shown on Page 3 of this data guide.
This is very useful during FCC testing to compensate for antenna gain or other
product-specific issues that may cause the output power to exceed legal limits.
A variable resistor can be used so that the test lab can precicely adjust the output
power to the maximun level allowed by law. The resistor’s value can be noted
and a fixed resistor substituted for final testing. Even in designs where
attenuation is not anticipated, it is a good idea to place a resistor pad connected
to LADJ and GND so that it can be used if needed.
Page 7
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