Microcontroller 8051 with RF-tx and rx and encoder and decoder

ashishdaga1 · May 23, 2008

guys help me out plz.i have set up a rf tx and rx using 8051 along with ht640 encoder and ht648l decoder,TX and RX-315Mhz from vegakit.
here is my circuit diagram
**broken link removed**
the problem is when i connect the 8051 to the encoder the data send is not received at the receiver but as soon as the 8051 connections are removed the transmisison works perfectly.
thus manually the RF communication works perfectly i.e the 8 bit data which i give manually at the encoder is perfectly send and then later received at the decoder side
Is some sort of interfacing required b/w encoder-ht640 and microcontroller 8051 or at the receiver side b/w the decoder-ht648l and 8051 .help me plz its urgent

ericgibbs · May 23, 2008

hi,
The datasheet for the ht648l decoder says its compatible with the MCU.

A common problem is, you may have set the MCU port pins as OUTPUTs within the program.?

Papabravo · May 23, 2008

In the 8051 architecture ports 1,2, & 3 are quasi-bi-directional ports. They have weak pullups, and no data direction registers. They require no configuration and have 1's written to them at reset making them inputs or "weak" 1's They will sink a couple of milliamps, but source only several tens of microamps.

Now here is the GOTCHA. Port 0 has OPEN DRAIN outputs. There is no weak pullup inside the chip, you gotta have external pullup resistors!

ashishdaga1 · May 23, 2008

Papabravo said:
In the 8051 architecture ports 1,2, & 3 are quasi-bi-directional ports. They have weak pullups, and no data direction registers. They require no configuration and have 1's written to them at reset making them inputs or "weak" 1's They will sink a couple of milliamps, but source only several tens of microamps.

Now here is the GOTCHA. Port 0 has OPEN DRAIN outputs. There is no weak pullup inside the chip, you gotta have external pullup resistors!

i have connected external pull up at port0 but problem is that the interfacing the transmitted data is not been able to decode.
i checked it separately also i.e only 8051 connected to encoder and TX and only the decoder and RX at the other end but the decoder is not able to read the data send by the 8051 through the encoder.
mic->encoder->tx rx->decoder
this does not seem to work also

is some sort of interfacing required to connect 8051 to another IC?????

Papabravo · May 23, 2008

Have you looked at the RF output with an oscilliscope? Can you get your hands on a working receiver to test the transmitter? Can you get your hands on a working transmitter to test the receiver?

blueroomelectronics · May 23, 2008

Just curious, is the transistor "From Tree" supposed to short VCC to GND?

ashishdaga1 · May 23, 2008

blueroomelectronics said:
Just curious, is the transistor "From Tree" supposed to short VCC to GND?

no its acting as an inverter so when "from tree" is vcc P3.6 is gnd and when "from tree " becomes gnd P3.6 is vcc.this is to monitor external events and i am checking p3.6 in my 8051.as of now this is not connected in my circuit but will be added later.presently i am concerned with
mic->encoder->tx

rx->decoder

the above does not seem to work so is any sort of interfacing required b/w ports of 8051 and the encoder

blueroomelectronics · May 23, 2008

Do you have an Oscilloscope or Logic analyzer? Even a logic probe or heaven forbid an 8051 ICE? What tools do you have for testing your circuit?

Papabravo · May 24, 2008

ashishdaga1 said:
no its acting as an inverter so when "from tree" is vcc P3.6 is gnd and when "from tree " becomes gnd P3.6 is vcc.this is to monitor external events and i am checking p3.6 in my 8051....

Then you dont want to connect the collector of the transistor to Vcc. Just connect it to P3.6. As I explained above there is an internal weak pullup that will pull P3.6 to Vcc when the transistor is off.

Papabravo · May 24, 2008

A cursory review of the HOLTEK HT640 Datasheet reveals the the TE pin on the HT640 is a CMOS input which is pulled low. This is problematic since the 8051 has outputs with a weak pullup. When the 8051 pin P3.3 is at a '1' this will form a voltage divider and the actual level on the P3.3/TE net will depend on the relative magnitude of the pullup and the pulldown. When P3.3 is low all will be fine and you should get a solid low.

I recommend that you insert a CMOS buffer between P3.3 and TE. I don't know if PORT 0 with external pullups might not have the same problem.

https://www.electro-tech-online.com/custompdfs/2008/05/MC74VHC1GT50-DPDF.pdf

Just to be super picky, downright arrogant, and altogether disagreeable I'd like to point out that your schematic has no pin numbers on it. How on earth would you ever be able to tell if the board was wired correctly?

blueroomelectronics · May 24, 2008

Well it does have the "Prakash" seal of approval

Papabravo · May 24, 2008

blueroomelectronics said:
Well it does have the "Prakash" seal of approval

Are they also super picky, downright arrogant, and altogether disagreeable?

I do so love the rhyme and meter of Kipling.
Bill, you can be the crocodile while I play the bi-colored python rock snake.

It just occurred to me that the same problem may occur on P3.2/VT with respect to the receiver. Time to check the datasheet.
EDIT: Nope VT is a plain old CMOS active high output.

blueroomelectronics · May 24, 2008

Why not connect the RF units directly to the microcontroller? What's the point of the HT640 & HT648?
**broken link removed**

Nigel Goodwin · May 24, 2008

blueroomelectronics said:
Why not connect the RF units directly to the microcontroller? What's the point of the HT640 & HT648?
**broken link removed**

I thought they were for non-micro-controller applications?.

Simpler to remove them and do Manchester coding/decoding in software.

blueroomelectronics · May 24, 2008

Nigel Goodwin said:
I thought they were for non-micro-controller applications?.

Simpler to remove them and do Manchester coding/decoding in software.

That's what I thought, saves a bucket of I/O too.

Papabravo · May 24, 2008

I guess the folks at "Prakash" have other ideas.
I did like the croc, although I had to work a bit to see him.

blueroomelectronics · May 24, 2008

Well I do like the rubber stamp, makes it seem so official.
**broken link removed**

ashishdaga1 · May 24, 2008

thanks all of u for ur replies.i think for driving ht640 u will need transistors as shown in the link.it did work with this driving transistors
**broken link removed**
is there any other alternative to these instead of these transistors

ashishdaga1 · May 24, 2008

also about not giving the pins really sorry for that but my main problem is interfacing 8051 with encoder/decoder.rest circuit i have taken care off.i did have pull-up 10k before at port 0 but it did not work so i tried with these transistors instead.

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