I like your "main.vi" instrument, but unfortunately, I think the AD7476A driver has not been designed with multiple ADCs in mind, so let's dig a little deeper.
"Declare Slave Select" command limits the value 1-7 because the SPI peripheral can drive directly only this amount of devices, each of them being validated in concordance with SPI_FLAG register. Notice that in DMA transfer mode (AD7476A driver case) CPHA =0 and SS pins are automatically selected at the beginning of each transfer: for each bit set to one in SPI_FLAG a low pulse will be generated at the corresponding pin! (These bits are set to one by Enable Slave Select or Declare Slave Select).
It would had been great if the reading would switch the slaves but is not.
I am hopping you'll not expect 1 MSPS for all ADCs. The conversion timing of AD7476A is determined by the serial clock: the driver sets the SPI clock about 16 times sample rate. That's why you have to give a higher value to the sample clock passed to BF AD7476A Control.vi:
For a system with 5 ADCs and 100 KSPS the value should be 500000. Notice that inter-channel delay is in this case 2 microseconds, so the time-difference between first sample of first channel and 5th channel is 8 us (almost equal with sample period) - this might be an issue in some applications.
However, more information can be accessed in hardware description (ADSP-BF537 Blackfin Processor Hardware Reference ), peripheral driver documentation (C:\Program Files\Analog Devices\VisualDSP 5.0\Blackfin\docs), and drivers source code (C:\Program Files\Analog Devices\VisualDSP 5.0\Blackfin\lib\src\drivers). Here I would just say that it seems circular buffer are not supported (see VisualDSP 5.0\Blackfin\docs \spi\adi_spi_int.pdf and adi_spi_dma.pdf), and move to some solutions:
1.Reconfigure the SPI to use interrupts and use callbacks to switch programmatically the slaves using BF SPI Control.vi "Select/Deselect Slave" (of course, each of the salves pins should be "Declare Slave Select" with BF AD7476A Control.vi or "Enable Slave Select" using BF SPI Control.vi). This will add some overhead and, depending on sample rate, you might lose some samples. Also there is a limit of only 7 slaves.
2.Set DMA for a single word and switch slaves in the callback after each conversion result (of course, this will have the same cons as #1).
3.Add a simple external hardware to select automatically ADCs (for example a counter+decoder clocked by one of the seven SS pins and reset using EIO). Then play with buffers to identify channels (e.g. 1D buffers with 5 elements for 5 ADCs). This will add less overhead, as callbacks will be triggered after one sample from all channels has been acquired (or even better after n samples, if buffer length is nx5, etc.)
In all above solutions you have to open and close AD7476a driver only one time in the "main.vi.
#1 and #2 looks like your solution posted previously but the command should be Select/Deselect for interrupts and Enable/Disable for DMA. Also the slaves should be switched exactly at the end/beggining of each received word.
Message Edited by NicB on 09-11-2009 06:21 AM
