Hi,

I am a hobbyist...  Sorry

I am trying to set up a circuit for an RF control of an LCD.  In setting up
the circuit I discovered that I could only send about a byte a second over
the RF link.  On removing the RF transmitter and RF receiver and attaching
the encoder HT640 straight to the decoder HT648L I discovered the problem is
not the RF devices, but the speed at which data is being encoded and
decoded.

History:

1) The circuit is driven by a PIC16F877...  I can vary the delay between
changes to the pins on the decoder from a single "nop" statement to a loop
of about a second.
2) I do have a led tied to the valid transmission output of the decoder.
3) For debugging purposes, I have tied my transmit enable on the encoder to
Vcc as long as the circuit has power.
4) Also, my LCD works when the transmission works.   (About 8 baud.)
5) My resistors are 390K between OSC1 and OSC2.
6) The circuit is driven off of a 5v breadboard.
7) Here is the datasheet for the encoder:
http://www.rentron.com/Files/ht-640.pdf
8) Here is the datasheet for the decoder:
http://www.rentron.com/Files/ht-648l.pdf
9) I recognize it would help if I sent the Pic program, but it is on a
dedicated pc for pic development that is not yet tied to the network.

Given that I do not have fancy equipment and given that I set up the circuit
as described by the documents on Reynold's web site.  Specifically:
http://www.rentron.com/remote_control/txlc-434.pdf

Here are my questions:

1) Is the 390K resistor appropriate to drive the ht640 at more than about 8
baud or is this just a demo chip for driving the RF device that can't really
take advantage of the device?  The data sheet says this is somewhere around
80KHz.  (As a side note, the RF device operates at a frequency of 434 MHz -
Is there any correlation?  Somewhere I read I should be able to get around
5000 baud out of these devices which should be plenty to drive the LCD.)

2) Why did Reynold Electronics pick 390K resistors?

3) What questions should I be asking to try to troubleshoot this circuit?

4) What experiments should I run to give me more information?

Any help is appreciated.

My gut instinct is that this is either:

a) "timing" issue between the PIC and the encoder.  (since the timing
between the pic and encoder doesn't alter the speed of what the
encoder/decoder actually does, I doubt this is the problem.)

b) resistor issue on the encoder and decoder.

c) encoder/decoder issue where they just aren't fast enough (This may be
it...  If I take 80kHz and divide by 33 as shown in the picture on the
datasheet and then by 12 I get 202 bits per second.  If there are 3 words
that get continuously sent between the encoder and decoder, I may have
somewhere around 8 baud....  I don't know, I am just a hobbyist.  Why would
Reynolds suggest using the HT640 if it cannot fully drive the RF devices?
Doesn't make any sense.)

Again, any help...

Thank you,
Fowler

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