W-Linx Technology TXE-418-KH Encoder Operation Power Supply Requirements, Data Inputs, Using Ladj

Figure 8: Encoder Flowchart

ENCODER OPERATION

POWER SUPPLY REQUIREMENTS

The KH Series transmitter internally utilizes		Power On
the HT640 encoder from Holtek. The
encoder begins a three-word transmission
cycle when the Transmission Enable line		Standby Mode
(TE) is pulled high. This cycle will repeat
itself for as long as the TE line is held high.
Once TE falls low, the encoder output	No	Transmission
completes its final cycle and then stops as
		Enabled?

The module does not have an internal voltage regulator; therefore it requires a clean, well-regulated 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; therefore, providing a clean power supply for the module should be a high priority during design.

Vcc TO

MODULE

10Ω

Vcc IN

+ 10μF

shown in the Encoder / Decoder Timing
diagram. When a transmission enable signal		Yes
is applied, the encoder scans and transmits		3 Data Words
the status of the 10 bits of the address code		Transmitted
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	No	Transmission
		Still Enabled?
individually preset to logic high, low, or
		Yes
floating. The floating state on the data input
is interpreted as logic low by the decoders		3 Data Words
since the decoder output only has two		Transmitted
states. The address pins are usually set to		Continuously
transmit particular security codes by DIP

switches or PCB wiring, while the data is selected using push buttons or electronic

switches. The floating state allows the KH transmitter to be used without pull-up or pull-down resistors on the data and address input lines.

Encoder
Transmit
Enable	< 1 Word
Encoder
Data Out		Transmitted Continuously		3 Words
	3 Words
				214 Clocks
	2 Words	214 Clocks
Decoder VT
	Check	Check
Decoder
Data Out
		1/2 Clock Time	1/2 Clock Time

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 (310) 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

A 10Ω resistor in series with the supply followed by a Figure 10: Supply Filter

10µF tantalum capacitor from VCC 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.

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

The module’s Transmit Enable (TE) line controls transmission status. When taken high, the module initiates transmission, which continues until the line is pulled low or power to the module is removed. In some cases this line will be wired permanently to VCC and transmission controlled by switching VCC 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.

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 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