SPI chip select between ADS1282 and DAC8563.

Ravi,

If you could provide a schematic or illustration of what you're describing that would make this a little clearer to understand. Please include the digital connections for the ADS1282 and the DAC8563 to your micro-controller.

Hi Ravi,

First off, welcome to the TI E2E forums!

Regarding your dilemma, you cannot use SYNC as a chip select on the ADS1282 as this does not disable the SPI interface. Using RESET for /CS may work (I haven't tested it) but it would be impractical as it would reset the register settings of the ADS1282; inconvenient if you don't use the default settings.

I would recommend MUX'ing the SCLK to both devices to "gate" the SPI communication as shown in the following attachment:

8535.ADS1281 and 1282 Shared SPI Schematic.pdf

This uses the same number of pins as you would if you had two SPI devices, however, it can support up to four devices.

Best Regards,
Chris

Dear Kevin ,
Thank you for your reply.

Unfortunately the design schematic is not complete yet. But I could describe the circuit.

1. It is a three electrode sensor system. In the analog part : DAC will be used to provide the potential difference between the reference electrode and working electrode.  ADC will be used to measure the changes at working.

2. The problem is with the digital part. The MCU has only one SPI. So I have to switch the SPI between the ADC and DAC.

3. As I understood from the data sheet of the The SYNC of DAC8563 could be used as a chip select. and the DAC will use only DIN.

4.But there is no chip select found in case of the ADS1282. AS DAC do not use the DOUT so I could use that directly without any hesitation for ADS1282. But I have to multiplex/Chip Select  the DIN only in case of the ADS1282.

Now I think you could understand the problem.

Dear Hall,

Thank you for your valuable reply.

I think it will better to select the DIN only rather than doing it for the SCLK.

Because the DAC8563 could be selected by the SYNC pin in it as far as I studied from the data sheet and also this will only uses the DIN only (not the DOUT). and the ADS 1282 could use the DOUT without any hesitation in my my case.

Now the problem is only with DIN of the ADS1282. If I could use a mux to control the DIN only for ADS1282, would it work?

Hi Ravi,

You could switch the DIN pin, but my preference would still be to switch the SCLK pin. If either part does not receive an SCLK, then it will not clock in the information on DIN (it will get ignored).

Conversely, if you switch DIN, you will always have a 0x00 (WAKEUP for ADS1282) or 0xFF (Undefined for ADS1282) command issued while talking to the other device, assuming you pull the disconnected DIN low or high, respectively.

In either case, you will need a pull-down or pull-up resistor on the disconnected lines so they don't float to unexpected values.

Regards,
Chris

Hi Hall,

One more doubt I have. If I switch the SCLK then the Register setting which I would have set previously would remain their in ADS1282 or each time I have to re-set / set the resistor setting. 

Thanks Mr Christopher Hall