Confusion over Holtek HT12A datasheet

[Brian Hoskins]

Hi everyone,

I wonder if anyone here is familiar with the Holtek range of encoders and in particular the HT12A?

Please see the datasheet for the device in my attachment. My confusion surrounds the transmit enable part of the device. According to the datasheet D8-D11 are used as the data inputs and transmit enable pins simultaneously. They are active low, so the device will transmit data when one of these pins is driven low. If a data pin is driven low briefly it will transmit one complete cycle. If a data pin is held low then it will transmit continuously. I've obtained this information from the pin description on page 3 and the transmission timing diagrams on page 5.

Now, here is where my confusion starts. The pin description on page 3 says that the data pins D8-D11 should be externally set to VSS or left open. This appears to contradict the transmit timing of the device, which shows that if a data pin is held low it will transmit continuously. If I don't want to transmit continuously, then surely I shouldn't be externally setting any of those pins to VSS?
The second part of my confusion surrounds data transmission. The only way to trigger the device is to set one of the data pins low. But what if I wanted to transmit 1111? How would I then trigger the device? Also, what if I only want to briefly transmit a nibble of data? I would have to set the data nibble on pins D8-D11 for a short time and then immediately tie them to 1111 afterwards to stop it transmitting. It all seems a little contradictory and silly?

Am I not understanding fully how this device is supposed to operate? My feeling is that the device is severely limited by using the data pins themselves as transmit enable pins. There should infact be a seperate transmit enable pin, as for the HT12E. Unfortunately I want to use the 38Khz carrier that is a feature of the HT12A so it's not simply a case of my choosing to use the HT12E instead.

If anyone can clear up my confusion, or agree with my conclusions then I would appreciate your response.

Brian

 

[Nigel Goodwin]

It's supposed to be a remote control transmitter, so you press the button and it sends a code - this is why it doesn't have a separate transmit line.

If you want things to be EXACTLY as you wish, program your own controllers, rather than using the pre-programmed Holtek ones.

 

[Brian Hoskins]

I completely agree with you, I am programming my own transmitter as a personal project. This project is part of a Uni assignment I'm working through.

If what you say is correct, then this device is only capable of transmitting 4 data codes. My assumption was that the data pins could be used in binary mode, allowing the transmission of 16 data codes.

I tell you what, I'm going to set one of these up in practice and see how it behaves. I've got most of the parts, all I need to do is rob a 455Khz resonator from an old remote.

Brian

 

[Nigel Goodwin]

Think diode gates! (or ROM), you can convert a single button press to a 4 bit code with a few diodes (minimum one, maximum four - for each button).

 

[Brian Hoskins]

I've built the entire thing and... well it's not working yet, but I am happier with the way it's "supposed" to work I think. 4 buttons for 4 functions, not 16 functions as I wrongly assumed before.

The transmit side appears to be working as far as I can tell, I think my receive side is the problem. I'm using a TSOP34838 as a receiver, and it's basically an IR receive package that is sensitive to the same wavelength as my IR LED and is designed to demodulate a 38Khz carrier. I think that's where my problem is though - either there's a problem with the IR receiver itself or maybe I shouldn't be demodulating the 38Khz at this stage - I'm not sure if the Decoder is expecting to see the 38Khz carrier or not.

In the datasheet under application circuits they don't show an example of a suitable IR receive stage, they just show it as a block diagram called "IR Receive Circuit".

Some more investigation needed I think.

Brian

 

[Brian Hoskins]

Hmmmm yeah. Maybe I should have been using a receive circuit comprised of a simple phototransistor, not a full IR receive package. Perhaps the decoder IC is expecting to demodulate the 38Khz itself.

Brian

 

[Nigel Goodwin]

Hmmmm yeah. Maybe I should have been using a receive circuit comprised of a simple phototransistor, not a full IR receive package. Perhaps the decoder IC is expecting to demodulate the 38Khz itself.

No, it's expecting a data input, either from a wireless module, or an IR receiver IC.

Someone here used them the other year, I think it might have been either Krumlink or Marks256?.

 

[Brian Hoskins]

Ok, good. Well I'm sure I'll get it working - I don't think I've ever done a project that I put together and had it work first time. If I do have a bad time with it I'll give those two a shout and ask them if they managed to get theirs working.

Thanks,

Brian

 

[Brian Hoskins]

Just thought I'd post an update incase you're interested Nigel (or incase anyone else is). I'm at work having a look at the datasheet for my IR receive IC and I've had a bit of a Eureka moment. The datasheet doesn't specifically mention that it's output signal is inverted but I was looking at the block diagram and it shows a transistor on the output used in common emitter mode. With the arrangement shown it is clear that the output signal will be inverted. I've then checked some timing diagrams and it shows the signal being inverted on there too - doh!!!

I didn't take that into account obviously. This discovery will be useful in my personal IR project as well. I don't want to bother using hardware to correct the inversion, I'll just write my software such that the inversion is taken into account.

For this project I probably will need to invert it in hardware though, and the fact that I haven't done that is probably why it's not working!!!

Brian

 

[Nigel Goodwin]

I would expect the Holtek chips to be designed for that?, they are the standard IR receivers, and they all are active low. But stick a transistor inverter between them and see what happens.

 

[slamslam123]

the data pins can be selected as you want, the TE must be low to transmit your date.
i understand above from data sheet.

 

[Brian Hoskins]

Thanks for the reply, but I got this working a long long time ago. I hadn't considered that the output was inverted - this isn't specifically mentioned in the data sheet, but it is shown via the IC internal block diagram and also the timing diagrams.

Thanks,

Brian

 

[Boncuk]

Hi Brian,

it's just an idea, but it might help you understand the operation of the Holtek devices.

This is a 4-channel-IR-relay circuit sold as a kit from "Anykits".

There is also an 8-channel remote available there, but the schematics must be purchased with the kit.

Regards

Hans

 

[Brian Hoskins]

Thanks for that Boncuk, reply appreciated, but this is actually a very old thread that seems to have been re-erected. I conquered my confusions about the datasheet for this device and got my project working quite some time ago!

Thanks again for the reply though,

Brian

 

[Boncuk]

Thanks for that Boncuk, reply appreciated, but this is actually a very old thread that seems to have been re-erected. I conquered my confusions about the datasheet for this device and got my project working quite some time ago!

Thanks again for the reply though,

Brian

Hi Brian,

may be you know a way how to control eight channels. Four is not enough for the kind of application I have in mind but eight will suffice.

Hans