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Playing around with the 23LC512-I/P SPI SRAM chip from Microchip, I found something strange in the datasheet (http://www.microchip.com/mymicrochip/filehandler.aspx?ddocname=en559704); on page 13, there are separate diagrams labeling pin 3 as NC in SPI and SDI mode, but a note at the bottom says pin 3 should not be left floating in SPI/SDI mode. I expect that tying this to GND is the right way to go, but should I wire it directly, or put a resistor in line?

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  • \$\begingroup\$ ww1.microchip.com/downloads/en/AppNotes/01484A.pdf application notes page 4 talks about what to do with unused input pins. Sounds like you can leave it floating, but they tell you not to in order to cover their tail. They say to pull to Vcc with 10k resistor. \$\endgroup\$
    – I. Wolfe
    Feb 2, 2015 at 19:41

2 Answers 2

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Take a look at Microchip's application note AN1484, entitled "Recommended Usage of Microchip 23XX512/23XX1024 Serial SRAM Devices"

On page 4, it says this:

Pin 3 – SIO2 (available on 23AXXX and 23LCXXX devices) is used for SQI mode of operation. As this is an input pin, this should not be left floating. This pin may be connected to either VSS or VCC. It is recommended that this pin be connected to VCC in the event that the device enters SQI mode, as this pin will need to be high to exit SQI. For the 23LCVXXX devices, pin 3 is internally tied high, so can be treated as a true No Connect.

For example, Figure 1 from the app note shows the recommended connections for SPI operation:

fig1

In your case, the upper schematic is appropriate (23LC512 has no VBAT support), so pin 3 should be tied to VCC via a 10K resistor. The lower figure is for 23LCV512, in which case it can be left floating.

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Looks like a typo to me. I imagine it should say "should be left floating", given all the NC right above it. Also, when something should not float, data sheets often say what you should do with the connection.

Of course, I have no way of actually knowing. If it's important, I suggest checking with a microchip engineer through customer support.

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