< I am using 8 ADS1256 adc's in a board and through FPGA reading the output to microcontroller on multiplexing basis
but whatever command I give ADC gives output as 00 even no data is coming on DOUT pin.
even I am not able to read configuration registers > <please give some hints to solve this problem>
You better start!
Give us hints what tests you have been performing. Does your fpga have correct programming? Is your read/write commands reaching the individual ADC? Are all signal pins on the ADC driven as they should? Can you write to the ADC?
initially i am just trying to read the configuration registers of the ADC. But every time it gives a different output sometimes it shows the values of five status register as 20 , 01 ,20 ,f0, E1 and next time other random values. can I find some simple option of a Low noise ADC (communicating through SPI) which is not as erratic as this one. even if i give inputs at its input pin the output is always faulty. ADc inputs are reaching ADC pin in a proper way that I have checked on oscilloscope. Actually I wanted to knoe the proceedure of giving right commands to this ADC. Like when to reset it when to give synch signal etc.
"can I find some simple option of a Low noise ADC (communicating through SPI) which is not as erratic as this one."
Unless you actually have a broken chip, this will not be the fault of the ADC itself; it must be something in your system: analogue hardware and/or digital hardware and/or software...
"sometimes it shows the values of five status register as 20 , 01 ,20 ,f0, E1 and next time other random values" How are you viewing those values? How are you sure that it's the "wrong" values being read from the chip, rather than some problem in the way you view the values?
You said you have 8 multiplexed ADCs - maybe your multiplexing is broken, and you're reading different values from different ADCs... As I said before, have you tried this on a simple system with no FPGA and only one ADC?
"Actually I wanted to knoe the proceedure of giving right commands to this ADC. Like when to reset it when to give synch signal etc."
Surely, that will be in the Datasheet?! You certainly need to get the fundamentals like this sorted out before you start messing with FPGAs, multiplexing, and other such stuff!!
You still haven't answered the previous questions:
Are you using a real 80C251 chip, or is it a "soft" core within the FPGA?
Is it really a '251, or is actually just a '51 or '52?
Especially if it's a "soft" core, are you sure that the processor itself is correctly configured and working? eg, have you done the basic LED flash and "Hello, World"?
microcontroller is not soft core. it's a hard core microcontroller.
in datasheet it is not given while reading or writing to status registers we have to give reset command or not. actually the board is already designed i cannot start from basic , like taking one ADC at a time , then check and all that. if the ADC is not having any problem and if I can view the input going at input pin why not the output is coming. According to datasheets the timing considerations I have taken care of.
before pointing to the ADC we have checked the FPGA programs and other sources from where fault may be caused.