Parallel SPIS peripherals without a TX data stream.

[henryh]

I am trying to use 2 SPIS peripherals to collect ADC data and have a couple of questions:

1.)  I do not need a MISO function in my SPIS implementation and would like to know how to configure that.   Can the peripheral be configured to act as an RX only device?  If so, can I use the SDK?

2.) Can the two peripherals share the same SCK pin?  (I assume the answer is NO but thought I might get lucky).

[awneil]

Why do you want 2 SPIS peripherals?

When you say "collect ADC data", that suggests that the ADC(s) is/are external - in which case it/they would usually be the slave(s) ?

Perhaps a diagram would help ... ?

No, 2 peripherals can't share the same pins.

[henryh]

I am using 2 external ADC's that have to run at a specific frequency which is related to the SCK input (and thereby locks the entire data collection process to that clock).  The '840 cannot generate this frequency so I am using an external clock source and enable logic to control the data collection process.  In this application, both the ADC's and the '840 SPIS are acting as slaves since neither generates the clock or enable with each ADC having a separate instance of SPIS.  There is no data return to the ADC therefore I would like to eliminate the TX buffer if possible (it is relatively large) and re-purpose the MISO pins.  I think the MISO pin can be eliminated using the "NOT USED" definition or simply changing the pin configuration after setting up  the SPIS peripheral but its not clear to me if the peripheral will operate without a TX buffer.

[awneil]

OK - now I see.

But not something I've tried.

[Edvin]

Hello,

What frequency do the ADC's run on? 

 

henryh said:

both the ADC's and the '840 SPIS are acting as slaves

 Then who is the master? Or how do you control the SPI CS/SS pins on the ADCs and the nRF?

BR,

Edvin

[henryh]

Another device controls the enable and clock generation under command from the '840; that device runs at a base frequency that is compatible with my desired sampling rate.   The '840 is designed to run off 64 MHz which for a variety of reasons is not able to provide the time base I need for this application.

[Edvin]

Hello,

The max speed for the SPI is 8MHz, so I don't think you will get it to work with 64MHz.

I was able to find this in the specification, but we are currently doing some merging on our documentation libraries, so I can't get the link to it. However, here is a screenshot:

It is found under nRF52840->product specification->peripherals->SPIS->Electrical Specification

BR,

Edvin

[henryh]

The ARM runs at 64MHz, I am running the SPI peripherals at 6.144 MHz.

[Edvin]

henryh said:

The ARM runs at 64MHz

 What ARM?

[awneil]

The ARM of the Nordic?

[henryh]

The Nordic processor runs at 64 MHz while I am running the SPI (and the ADC's) off a 6.144 MHz clock that is required to meet the signal processing needs of the application.