ADS7843 Touch Controller

[wgenik]

Hello!

For the past few years I've been experimenting with electronics and have learned some of the basics, mostly from the internet and great sites like this one. Now I've come up against a project that's stretched past my abilities, so I figured I'd ask for some help.

I'm trying to build a touch controller for my 4-wire resistive touch screen LCD. I've got some Texas Instruments ADS7843 chips which are supposed to be able to convert the resistive touch screen to a serial output, using something similar to the following schematic (from datasheet)...

Now, as far as I can tell, I still need another chip to actually interface with my PC's serial (RS232) port. Enter the TI MAX232 chip, which can be hooked up something like this...

So how do I interface the two? On the MAX232 looks like I need to supply TXD and RXD connections, but the ADS7843 chip has quite a few outputs:
- Serial/Conversion Clock
- Chip Select
- Serial Data In
- Converter Status
- Serial Data Out
- Pen Interrupt

I'm guessing that the TXD and RXD basically get connected to the Serial Data In and Serial Data Out. What about the other 4, Serial/Conversion Clock, Chip Select, Converter Status, Pen Interrupt?

Do I need some other chip as well, such as a microcontroller?

Thank you very much for anyone's input, and sorry if the images don't show up properly... Hopefully I've provided all the relevant information, but if you need to know anything else, please ask!

Warren

 

[Nigel Goodwin]

It's probably easier to use a microcontroller to interface to the PC, or you could use the PC parallel port to connect the chip direct - the serial port isn't suitable as it's asyncronous, and the chip is syncronous plus other control lines.

 

[wgenik]

OK I was under the impression that this ADS7843 chip was only serial. Any idea how I can hook it up parallel then?

 

[Nigel Goodwin]

OK I was under the impression that this ADS7843 chip was only serial. Any idea how I can hook it up parallel then?

You don't really - you write a program on the PC to create a syncronous serial port, with extra switching lines, to use the parallel port to control the IC. PIC programmers commonly work in a similar way.

If you used a PIC to interface it to a serial port, you would be writing a similar program in the PIC - there should be examples in the datasheet?.

 

[wgenik]

Actually, I had no plans of using a microcontroller/PIC at all, unless you are referring to the MAX232 which is (AFAIK) just an IC to switch the levels to something usable by the PC's RS232 port, and not a PIC. Is that what you meant? Or am I completely missing something?

 

[Nigel Goodwin]

You need 'something' programmable, in order to drive the chip - like I said, you could use the parallel port and program the PC to do it. The MAX232 is just a level shifter and inverter.

 

[gramo]

I would consider using the DS275 for interfacing with your PC, it requires no additional components, and it’s dead simple to connect to your PC.

You can’t really just use a micro controller to interface with a PC as the data lines on a COM port are generically + - 12V. I believe this is different for parallel ports, but many computers don’t come with these any more.

As practically all logic devices run on 3.3 or 5V, you’re going to damage either your motherboard or your micro if you don’t use a device to interface the two.

For an example on how to connect it to your PC to a PIC Micro & a visual basic program that shows how to setup up a COM port for serial data transfer, have a look at this;

**broken link removed**

 

[Nigel Goodwin]

I would consider using the DS275 for interfacing with your PC, it requires no additional components, and it’s dead simple to connect to your PC.

But it basically just a MAX232 varient.

You can’t really just use a micro controller to interface with a PC as the data lines on a COM port are generically + - 12V. I believe this is different for parallel ports, but many computers don’t come with these any more.

I thought you were a PIC programmer?, all you need is a single resistor to connect a PIC directly to the serial port of a PC.

As practically all logic devices run on 3.3 or 5V, you’re going to damage either your motherboard or your micro if you don’t use a device to interface the two.

Hence the single resistor!.

For an example on how to connect it to your PC to a PIC Micro & a visual basic program that shows how to setup up a COM port for serial data transfer, have a look at this;

**broken link removed**

Have you not read the thread at all?, how would that help him?.

 

[wgenik]

Thanks again for the info. So I should be using either a PIC or the PC's parallel port... hmmm well this touch controller is going to go into a CarPC system, of which the graphic LCD is parallel, so I don't have a spare LPT port. And I've always wanted to get into PIC programming, so perhaps someone could suggest an inexpensive PIC / programmer that would get me started and do the job?

 

[evandude]

I just wanted to point one thing out, which seems to have been skirted around but not directly addressed:

Your confusion regarding the serial interface of this chip is understandable. The main point is that there's a difference between 'a serial interface' and the particular type of serial interface that a computer's serial port provides (RS232). Technically "serial" is a very broad term that can cover a lot of different communication protocols, but because people just say computers have a 'serial port' it's easy to assume that if any device has 'a serial interface' it'll be compatible with the computer's serial port, but that's not the case.

 

[wgenik]

Yeah, come to think of it, I've had this conversation before regarding "serial" not necessarily meaning "RS232". Wouldn't it be nice if serial things just plugged straight into all other serial things... Maybe if I look hard enough, I can find a touch controller chip with direct USB out!

 

[evandude]

Maybe if I look hard enough, I can find a touch controller chip with direct USB out!

Well, they are out there. I believe generally you'd be looking at a prebuilt controller board, and the associated chips are likely to just be microcontrollers programmed with proprietary firmware, and probably not extremely inexpensive.

In all reality, it shouldn't be extremely challenging to make one with a USB-capable PIC, for someone who already had the USB interfacing experience, but starting from scratch, you'd be in way over your head.

 

[gramo]

I thought you were a PIC programmer?, all you need is a single resistor to connect a PIC directly to the serial port of a PC.

Your still putting the output of the PIC/5V device in 'reverse bias' and current is traveling in the wrong direction, the only thing your doing by putting a single resistor in series with the PIC is reducing this current (This is keeping in mind that a standard COM Port Utilizes + - 12V Logic states).. Is that ok to do?? Ultimately you could still be doing internal damage to the logic device (PIC/other 5V peripherals)

Have you not read the thread at all?, how would that help him?.

It was in regards to wiring up a device to a PC for serial comms and the Visual basic program illustrates how to interface with your com control objects in VB

It was in response to the following:

So how do I interface the two? On the MAX232 looks like I need to supply TXD and RXD connections, but the ADS7843 chip has quite a few outputs

 

[Nigel Goodwin]

Your still putting the output of the PIC/5V device in 'reverse bias' and current is traveling in the wrong direction, the only thing your doing by putting a single resistor in series with the PIC is reducing this current (This is keeping in mind that a standard COM Port Utilizes + - 12V Logic states).. Is that ok to do?? Ultimately you could still be doing internal damage to the logic device (PIC/other 5V peripherals)

PIC's have protection diodes across MOST of the I/O pins, all you need is a single current limiting resistor and you can feed RS232 directly to the pin. The other way you can just connect the PIC pin directly, 0-5V works fine feeding the PC serial port.

However, as there's no hardware inversion, you MUST use a software UART rather than a hardware one. The BASIC STAMP uses this exact technique.

It was in regards to wiring up a device to a PC for serial comms and the Visual basic program illustrates how to interface with your com control objects in VB

You only read a line or two, none of which were at all relevent, he was wanting to interface a chip directly to a PC via the serial port, which wouldn't work as it's not an asyncronous device.

 

[gramo]

PIC's have protection diodes across MOST of the I/O pins, all you need is a single current limiting resistor and you can feed RS232 directly to the pin. The other way you can just connect the PIC pin directly, 0-5V works fine feeding the PC serial port.

Learn something new every day! I've always used the DS275, a single chip interface cheap solution, and after reading a few forums out there about how people have blown PIC's/Motherboards directly interfacing with their PC, I wasn't to keen to try it.

You only read a line or two, none of which were at all relevent

I'm good at doing that sometimes, I mean well though

 

[Nigel Goodwin]

Learn something new every day!

Something very OLD, not new

If you can find them anywhere, Parallax used to have application notes for early PIC's (using their own silly assembler), one of those showed how to interface with a single resistor.

If you're using a UART though, you need inversion, so you may as well use a proper chip, which is 'nicer' anyway!.

 

[wgenik]

Hello again guys! I've been doing more research, and since I can't find any LPT/COM circuits specifically regarding my ADS7843 (touch controller) IC, I've been looking for information on similar (in my noobish opinion) ADC's interfaced either to parallel port or serial port.

So here's what I've got so far:
http://www.epanorama.net/circuits/ad_serial.html
This link talks about connecting a **broken link removed**, which is a 8-bit ADC (mine is supposedly 12-bit or 8-bit), directly to the RS232 port. It's using some diodes and caps and apparently gives the clock signal (via programming) from the serial. Is this something that I'd likely be able to do with my ADS7843?
**broken link removed**


Or, **broken link removed** talks about connecting a 20-bit AD7703 using a crystal for clock cycles. I notice that the schematic for this circuit, however, doesn't have a "BUSY" (Converter Status), but rather a "DRDY" (Ready) pin. Are these related?
**broken link removed**


In my googles, I've (sort of) learned that the CS (Chip Select) pin is just to turn the chip on, so in my case of only using one chip might as well always be connected. True?

Thanks for everyone's help!

(apologies if images are too big or something)

 

[wgenik]

Anybody?

 

[Nigel Goodwin]

If you want to try it, then try it - personally I wouldn't use the serial port in that way - far better to use a PIC to control it (you have far more accuracy in PIC I/O than Windows PC I/O), and transfer the data via RS232.

 

[wgenik]

OK, so if I was to use a microcontroller, would a Texas Instruments MSP430-generation micro work? I have some samples of a few different models. How do they compare to Microchip's PIC line?