LTC2400 SPI interface

[PUGALENTHI.P]

Hai friends,

am new to this world of processors,but started playing with the PIC 16f690.
for now am trying to communicate with LTC2400-24 bit ADC to the pic through the spi interface but i dont know where to start from.
please help me friends and thanks for reading the post.

PUGAL

[mackrackit]

I have not used that part but a quick look at the data sheet says you should be able to use the SHIFTIN command in Pic Basic Pro.

You posted in PIC BASIC. Is that what you are using or are you using the Pro version?

The 690 also has a built in ADC.

[PUGALENTHI.P]

I have not used that part but a quick look at the data sheet says you should be able to use the SHIFTIN command in Pic Basic Pro.

You posted in PIC BASIC. Is that what you are using or are you using the Pro version?

The 690 also has a built in ADC.

am using BASIC and oshonsoft compiler.
i tried using the internal adc and its worked but i want to communicate with external adc using spi interface.please help me my friend.

[mackrackit]

This is an example for EEPROMS
http://melabs.com/resources/samples/x1/pbp/spix.htm
You should be able to modify it for the ADC

[PUGALENTHI.P]

why the command set changes even all the codes are in basic language.In the article they are using shiftin and shift out but my compiler uses shiftright and shiftleft,why such a difference is needed.why am asking this is because they are using "sck" and i dont know what for they are using.as i mentioned above am new to this world(MPU), so i dont think you will laugh at me.

[mackrackit]

Most of us here use this to compile
http://store.melabs.com/prod/pbp.html
I am not familiar with oshonsoft compiler. I did not notice that you were using that earlier.

Basically you will have three pins on something like this.

DATA
CLOCK (SCK)
LATCH or Chip Select

[HenrikOlsson]

Because you are using the Oshonsoft compiler and "we" at this forum are using the MELABS PicBasicPro compiler (or PicBasicCompiler) - not the Oshonsoft BASIC compiler.

You need to "translate" the any examples not written for your compiler so the instructions matches your compilers instruction set. BASIC is not known for being the most standardised programming language. Most i spretty similar in the basic command set like GOTO, GOSUB, FOR-NEXT, WHILE_WEND etc but when it comes to all the "special commands" like SPI, PWM, LCD, etc etc each compiler has it's own way of handling it. There simply is no standard for those type of commands.

A quick look at the Oshonsoft site seems to indicate that what you are looking for is the [url=http://www.oshonsoft.com/picbasiccompilerreferencemanual.html#677] SPI Communication commands. Not ShiftLeft and ShiftRight - they are for shifting bits around in memory, not for shifting them in or out a pin.

Don't worry about beeing new to this, we've all been there at one time or another. However, you may get better help with your project by posting on the forum dedicated for the compiler you are using. Good luck!

/Henrik.

[PUGALENTHI.P]

Because you are using the Oshonsoft compiler and "we" at this forum are using the MELABS PicBasicPro compiler (or PicBasicCompiler) - not the Oshonsoft BASIC compiler.

You need to "translate" the any examples not written for your compiler so the instructions matches your compilers instruction set. BASIC is not known for being the most standardised programming language. Most i spretty similar in the basic command set like GOTO, GOSUB, FOR-NEXT, WHILE_WEND etc but when it comes to all the "special commands" like SPI, PWM, LCD, etc etc each compiler has it's own way of handling it. There simply is no standard for those type of commands.

A quick look at the Oshonsoft site seems to indicate that what you are looking for is the [url=http://www.oshonsoft.com/picbasiccompilerreferencemanual.html#677] SPI Communication commands. Not ShiftLeft and ShiftRight - they are for shifting bits around in memory, not for shifting them in or out a pin.

Don't worry about beeing new to this, we've all been there at one time or another. However, you may get better help with your project by posting on the forum dedicated for the compiler you are using. Good luck!

/Henrik.

Hai brother,
for now i understood the differences among the instruction set on various compiler because of your reply and thanks for that.i want you to suggest me a compiler which will be better for future developments,because i think am still a wet clay and can be shaped.currently my project is communicating a pic16f690 to a external adc(LTC2400),I hope you will support me.THANKS A LOT FOR YOUR FURTHER SUGGESTIONS.

[PUGALENTHI.P]

Most of us here use this to compile
http://store.melabs.com/prod/pbp.html
I am not familiar with oshonsoft compiler. I did not notice that you were using that earlier.

Basically you will have three pins on something like this.

DATA
CLOCK (SCK)
LATCH or Chip Select

i now understood the use of "sck" that it is used to control the serial clk (pin) of the adc,but in the oshonsoft i cant find any command to do that.it simply got spicson and spicsoff to control the "cs" pin of the adc and no more to control the spi clk.please give me a good idea to do my project.As i mentioned above
my project is to communicate the LTC2400 to a pic 16f690 and to display on a 16*2 lcd.The lcd portion working good.THANKS FOR NOW AND THANKS A LOT FOR YOUR FURTHER SUGGESTIONS.

[mackrackit]

I would give the PBP demo a try if I were you
http://melabs.com/pbpdemo.htm

[PUGALENTHI.P]

I would give the PBP demo a try if I were you
http://melabs.com/pbpdemo.htm

thanks for the compiler,but when i bought the pic kit 2 it comes with that compiler.ill try to utilize that.how to handle the timing feature on the adc-LTC2400,do you have any idea?if have then please give me a sample code to get the data from the adc in picbasic.i connected the ltc2400 in 3 wire communication mode.thanks a lot for spending your valuable time with me.

[mackrackit]

The ****IN command in PBP will handle the timing for you when the correct MODE is selected.

The best I can do at the moment because I have not used the ADC you have is to have you look at the sample I linked you to and compare that to the data sheet of your ADC.

But I have to ask... Why use the external when the 690 has an ADC built in?

[PUGALENTHI.P]

The ****IN command in PBP will handle the timing for you when the correct MODE is selected.

The best I can do at the moment because I have not used the ADC you have is to have you look at the sample I linked you to and compare that to the data sheet of your ADC.

But I have to ask... Why use the external when the 690 has an ADC built in?

i have already tried the internal adc but for now my project is to read (4-20mA)0-5v from a pressure transduser(0-100bar) and then conver it into pressure units .so i need more accuracy and i want to use 24 bit adc.thanks for your support.

[HenrikOlsson]

Hi,
I'm sure you have your reason but can you explain why you need 24bits?

I mean, 100bar across 24bits gives you a resolution of 0.000006bar. Does a transducer capable of measuring 100bar really resolve to that kind of precision?

If you already have your 4-20mA converted to 0-5V (and not 1-5V) you'd get ~0.1bar resolution with the built in 10bit ADC. You could do some oversampling to "emulate" a 12bit ADC giving you ~0.025bar resolution.

Just some ideas.

/Henrik.

[PUGALENTHI.P]

Hi,
I'm sure you have your reason but can you explain why you need 24bits?

I mean, 100bar across 24bits gives you a resolution of 0.000006bar. Does a transducer capable of measuring 100bar really resolve to that kind of precision?

If you already have your 4-20mA converted to 0-5V (and not 1-5V) you'd get ~0.1bar resolution with the built in 10bit ADC. You could do some oversampling to "emulate" a 12bit ADC giving you ~0.025bar resolution.

Just some ideas.

/Henrik.

THANKS FOR YOUR SUGGESTION MR.HENRIK

Actually as you said the 10 bit adc gives a 0.1 resolution on 0~100bar range which is as equal to a "primary master" and thats fine,but i want to use the 24bit adc because i want learn how to communicate a external adc through "spi" to the pic and particularly the 24bit adc is because thats the one i have in my hand to give a try.can you please explain me about the "oversampling to "emulate" a 12bit ADC to give a ~0.025bar resolution"
THANKS A LOT FOR SPENDING YOUR VALUABLE TIME FOR ME.

[HenrikOlsson]

Hi,
I'd say learning qualifies as a (very) valid reason ;-) By all means try using the 24bit ADC but perhaps you should start with interfacing to something "easier" like a EEPROM to begin with so you learn the basics about how the SPI interface works. I haven't looked at the datasheet for your ADC though, perhaps it's pretty straight forward.

As for the oversampling, all you really do is take a bunch of readings and then average them. If you take 16 readings with your 10bits ADC, add them all together and divide the result by 4 you'll get a value ranging from 0-4096.

You can read a bit more about here. Note, you can not use that code with Oshonsoft, the theory behind it obviously applies to any language/compiler though.

Good luck!
/Henrik.

[PUGALENTHI.P]

Hi,
I'd say learning qualifies as a (very) valid reason ;-) By all means try using the 24bit ADC but perhaps you should start with interfacing to something "easier" like a EEPROM to begin with so you learn the basics about how the SPI interface works. I haven't looked at the datasheet for your ADC though, perhaps it's pretty straight forward.

As for the oversampling, all you really do is take a bunch of readings and then average them. If you take 16 readings with your 10bits ADC, add them all together and divide the result by 4 you'll get a value ranging from 0-4096.

You can read a bit more about here. Note, you can not use that code with Oshonsoft, the theory behind it obviously applies to any language/compiler though.

Good luck!
/Henrik.

hai Henrik,

Thanks for your suggestions.is that the communication between the eeprom and pic is very easy?what i thought is am trying to make use of the spi communication only and
no matter whether it is ADC or EEPROM (am i right).

when i used the internal adc for example
i injected 1.234v where the range is 0~5v and the related 10bit adc value is 252.7232 but the pic reads only 252 i took a bunch of readings and calculated the mean value (asume the value is same 252)
then i multiplied the value by 10000 then the value is 2520000 again divide it with 2048 and the answer is 1230.46 where the pic read only 1230
that means we get 1.230
as you said (252*16)/4=1008 /819.2=1.230

what is the difference between these two.

thanks a lot for clarifying my doubts

PUGAL

[HenrikOlsson]

Hi,
It's because the reading you get from the ADC most likely won't be stable at 252 across several consecutive samples, it will probably "tip over" towards 253 a couple of times.
Let's say you get the following values:
252, 253, 252, 253, 253, 252, 252, 253, 251, 253, 252, 253, 253, 252, 252, 252

The sum is 4039. Averaging them across 16 samples results in 252.4375 but you'll only "see" 252 which is ~0.29% off. Averaging them across 4 instead gives you 1009.75, you'll only "see" 1009 which is ~0.19% off.

Had the result been 1040 instead, averaging them across 16 samples would give you the same result, ~0.29% off while the oversampling method would only be 0.009% off.

Try it out and make sure you read thru the page I linked to, also look up the term oversampling. Here's a quote from Wikipedia:

In practice, oversampling is implemented in order to achieve cheaper higher-resolution A/D and D/A conversion. For instance, to implement a 24-bit converter, it is sufficient to use a 20-bit converter that can run at 256 times the target sampling rate. Averaging a group of 256 consecutive 20-bit samples adds 4 bits to the resolution of the average, producing a single sample with 24-bit resolution.

/Henrik.

[PUGALENTHI.P]

Hi,
It's because the reading you get from the ADC most likely won't be stable at 252 across several consecutive samples, it will probably "tip over" towards 253 a couple of times.
Let's say you get the following values:
252, 253, 252, 253, 253, 252, 252, 253, 251, 253, 252, 253, 253, 252, 252, 252

The sum is 4039. Averaging them across 16 samples results in 252.4375 but you'll only "see" 252 which is ~0.29% off. Averaging them across 4 instead gives you 1009.75, you'll only "see" 1009 which is ~0.19% off.

Had the result been 1040 instead, averaging them across 16 samples would give you the same result, ~0.29% off while the oversampling method would only be 0.009% off.

Try it out and make sure you read thru the page I linked to, also look up the term oversampling. Here's a quote from Wikipedia:

/Henrik.

HAI HENRIK,
thanks for your support.as you said i tried the oversampling method and it gave me a better result than the previous method i used.but i have a transduser which range from 0~60000 psi on 4~20 mA,i used a 250 ohm resistor and i got 1~5v difference.

in this case do i need to use a external ADC more than 14bit resolution or can able to use the oversampling method on internal ADC by sampling 4096 times and divide by 64 to get a 16bit resolution.is it possible? or do we have any other ways to achive it.
THANKS FOR YOUR KIND SUPPORT,
PUGAL.

[HenrikOlsson]

Hello,
You still haven't said what kind of resolution you actually NEED, just that you wanted to use 24bits becaue that is what you had. I also asked you earlier if you had the 4-20mA signal converted to 0-5 or 1-5V. With 1-5V it means that you are basically "wasting" 20% of the ADC range.

Now, with a 1-5V signal I'd configure the ADC to use the external Vref inputs instead of the VDD and VSS. That way, if you feed a stable 1V signal into Vref- and a stable 5V into Vref+ you'll use all the available resolution and you'll get 0 for 1V and 1023 for 5V. That way you get a basic resolution of 0.1bar as I outlined earlier and can increase that by oversampling.

If you try the aproach of 1024 samples remember that you need to add them into a 32bit variable as the result may not fit in a 16bit variable.

/Henrik.

[PUGALENTHI.P]

Hello,
You still haven't said what kind of resolution you actually NEED, just that you wanted to use 24bits becaue that is what you had. I also asked you earlier if you had the 4-20mA signal converted to 0-5 or 1-5V. With 1-5V it means that you are basically "wasting" 20% of the ADC range.

Now, with a 1-5V signal I'd configure the ADC to use the external Vref inputs instead of the VDD and VSS. That way, if you feed a stable 1V signal into Vref- and a stable 5V into Vref+ you'll use all the available resolution and you'll get 0 for 1V and 1023 for 5V. That way you get a basic resolution of 0.1bar as I outlined earlier and can increase that by oversampling.

"If you try the aproach of 1024 samples remember that you need to add them into a 32bit variable as the result may not fit in a 16bit variable".

/Henrik.

hai HENRIK,

THANKS FOR YOUR SUPPORT and sorry for delayed reply.am using "long variable" so the 32bit variable fits in it.how to feed 1v? is that any voltage regulator available for 1v.what about the -ve side.

as you said i used the internal adc and the oversampling method and got stable reading,but for now when i seperately send the readings to the lcd and hyperterminal it works but when i used both it doesn't work .i dont know what is the problem can you help me please.

THANKS AND REGARDS,
PUGAL

[HenrikOlsson]

how to feed 1v? is that any voltage regulator available for 1v.what about the -ve side.

Well, anyway you like really. The simplest way is a voltage divider but then the stability and accuracy of you reading depends on the stability of the powersupply rail, which it does when you're using the Vdd/Vss for Vref anyway.

The voltage supplied to Vref- is what will equal a returned value of 0 from the ADC, the voltage supplied to VRef+ is what will equal a returned value of 1023 from the ADC. So by feeding 1V to Vref- and 5V to Vref+ the ADC's 10 bits gets divided across 1-5V instead of 0-5V as is the case when you use Vss/Vdd as reference. (As long as you use 5V supply of course). Read up on the ADC section of the datasheet for details about the ADC.

I can't help with your problem as I have absolutely no idea how you've coded it.

/Henrik.

[PUGALENTHI.P]

Well, anyway you like really. The simplest way is a voltage divider but then the stability and accuracy of you reading depends on the stability of the powersupply rail, which it does when you're using the Vdd/Vss for Vref anyway.

The voltage supplied to Vref- is what will equal a returned value of 0 from the ADC, the voltage supplied to VRef+ is what will equal a returned value of 1023 from the ADC. So by feeding 1V to Vref- and 5V to Vref+ the ADC's 10 bits gets divided across 1-5V instead of 0-5V as is the case when you use Vss/Vdd as reference. (As long as you use 5V supply of course). Read up on the ADC section of the datasheet for details about the ADC.

I can't help with your problem as I have absolutely no idea how you've coded it.

/Henrik.

As you said i looked at the pins and i cant find any -vref.

"The positive voltage reference can be either VDD or an external voltage source. The negative voltage reference is always connected to the ground reference" this is what i found on the datasheet.

THANKS and REGARDS,
PUGAL

[HenrikOlsson]

Hi,
If that what it says then that's how it is. I didn't check the datasheet and I've never personally used a PIC where only the +Vref is available. Now I know they exist as well, thank you!

Then it gets trickier, one way is to use some analog circuitry to first subtracst 1V from the signal and then amplify it with a gain 1.25, that will give you 0-5V output. An easier way is probably to get a PIC which has it's -Vref available to the outside.

There are IC's available specifically for converting current loops to voltage, here's a MAXIM app note showing one.

But again, what kind of resolution do you really need? Do you have a datasheet for the transducer you're using?

/Henrik.

[PUGALENTHI.P]

Hi,
If that what it says then that's how it is. I didn't check the datasheet and I've never personally used a PIC where only the +Vref is available. Now I know they exist as well, thank you!

Then it gets trickier, one way is to use some analog circuitry to first subtracst 1V from the signal and then amplify it with a gain 1.25, that will give you 0-5V output. An easier way is probably to get a PIC which has it's -Vref available to the outside.

There are IC's available specifically for converting current loops to voltage, here's a MAXIM app note showing one.

But again, what kind of resolution do you really need? Do you have a datasheet for the transducer you're using?

/Henrik.

hai HENRIK,

THANKS FOR YOUR KIND HELP.For now i don't have the datasheet,but give some time ill find the datasheet and forward to you.Actually i want to read the 4-20 mA from a transduser with the accuracy of 0.1% and the resolution is 0.0000,the transduser has the accuracy of 0.05% and the range is 0-100,000psi this the maximum that we have.so if i manage to do the project,then i can use the same for all the other transduser with better accuracy.some are 0-5v and few are 0-10v which will be easy to do as you said.

THANKS and REGARDS,
PUGAL.

[HenrikOlsson]

Hi,
I'm not following you on the resolution. You say the transducer measures 0-100000 PSI with a resolution of 0.0001 PSI? That's 1.000.000.000 "counts" across the 16mA output swing - WOW!

I mean, even at 1 PSI "per count" the difference in output current is 0.016/100000=0.16uA. Push that thru your 250ohm resistor and you get a voltage change of 40uV per PSI - that's not a lot.

If you mean 100PSI then it gets a little more managable as 1PSI now eqauls a change in current of 160uA and a voltage change across the resistor of 40mV.

/Henrik.

[PUGALENTHI.P]

Hi,
I'm not following you on the resolution. You say the transducer measures 0-100000 PSI with a resolution of 0.0001 PSI? That's 1.000.000.000 "counts" across the 16mA output swing - WOW!

I mean, even at 1 PSI "per count" the difference in output current is 0.016/100000=0.16uA. Push that thru your 250ohm resistor and you get a voltage change of 40uV per PSI - that's not a lot.

If you mean 100PSI then it gets a little more managable as 1PSI now eqauls a change in current of 160uA and a voltage change across the resistor of 40mV.

/Henrik.

THANKS FOR YOUR REPLY


Sorry, actually the circuit must support wide ranges of transduser so that for low range transduers like 0~100 mbar and mid range transdusers like 0~500 bar it must have 0.0000 resolution and for high pressure transdusers as i said before like 0~100,000 it must have the accuracy of 0.1% and no need to worry about the resolution. so the circuit must have the capability to measure as much as possible.but for now i used the over sampling method and i got the accuracy of 0.1% for 0~100,000 psi on 10 bit internal adc which is my first stepping stone with the valuable help from MR.Henrik and thanks a lot for that.

THANKS and REGARDS,
PUGAL.