RS-232 SPI CONTROLLER – part 2.1

Someone asked me via PM, why would I use a unidirectional (i.e. one-way) SPI interface? It turns out that in my use of SPI interfaces, most of the devices I’ve had to interface with fall in this category. The times I’ve required a bidirectional interface are not as many; and most (not all) happen to be more complex applications. I am including a couple of more applications where a unidirectional SPI interface is useful. Once the interface circuit is available, writing a UI interface is pretty simple and can help debugging. Below is a UI to control the AD9834 DDS from Analog Devices. This is a very nice little IC with quite a simple interface. In the example below, using a 20MHz clock source (for my application it happened to be a TCXO – but that’s not relevant for this), the DDS can be used to synthesize anything from 0Hz to 10MHz with a really nifty resolution. One can even switch between frequencies, control the phase of the signal, sweep from one frequency from another, etc. The point is, once you have the simple SPI interface controller, you can play with the device any way you desire. One could point out that ADI already has an evaluation board for this IC. Even though that is true, that costs much more money than the simple SPI interface controller and is cannot be tailored as easy (and how about the other similar devices which do not have an eval board).

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For another example (perhaps simpler), a UI to control the MCP4921 DAC from Microchip is shown. This is simple DAC, again with a simple interface. In the example below, one can control the DAC to generate any voltage from 0V, to Vref. Using the x2 Gain option, the output can be modified to go from 0V to 2xVref (for applications when Vref is Vdd/2 or less).

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So what applications have I encountered where a bidirectional SPI interface is required? Reading sensors with SPI interfaces: ADC, Temperature, etc. Also some very specific (read proprietary) applications where there are ASICs or similar programmable devices meant to control many instruments and provide the communication interface.

All the UIs were done using Real Software; pretty cool software (for me anyways). Again, the Saleae Logic was used to validate the work; and the of course the actual use on the specific targets.