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I am new to electronics, but always wanted to build my own devices and decided to start with simple usb flash drive as a first project. I found some reference designs by cypress and it confused me a bit.

https://www.cypress.com/documentation/reference-designs/cy4665-cypress-upek-reference-design-usb-flash-drives-ufd here is the link to reference design.

So the problem is that in this reference design some active low pins are not pulled up at all, so they are basically left floating. For example "Read Enable" (RE) pin and "Write Enable" pins are left floating.

In my project i am using micron NAND flash https://datasheet.octopart.com/MT29F2G16ABAEAWP%3AE-Micron-datasheet-11549997.pdf (the datasheet) and cant find information regarding which pins are in what mode. maybe they are internally pulled up ? maybe pulled down. i am having a trouble finding it.

Maybe somebody can point me to another reference design, or explain why this pins are left floating.

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  • \$\begingroup\$ it is not only BGA package. but yeah, i know that high density NAND flash in bga package is not the best starting point but question is regarding pulling up active low pins on the chip, not about BGA package and board manufacturing. \$\endgroup\$ Commented Apr 7, 2020 at 19:17
  • \$\begingroup\$ Do you understand the differences between the Cypress part and the Micron one? I'm not going to download a 50mb project, try to convert the drawings and then do that for you. It would help for you to add a reference to the Cypress flash memory device they used and a snippet of the schematic showing that. Make sure it doesn't show it being pulled up/down somewhere else on the schematic (possibly a different sheet). \$\endgroup\$ Commented Apr 7, 2020 at 19:29
  • \$\begingroup\$ And even if you use the other package type (which I forget at the moment), it still isn't a "beginner" device. You need a 4 layer (minimum) board to use it, then you have to worry about controlled impedence, length matching, and issues with coupling high-speed signals. I'm using a similar Micron device in one of my projects and it requires some care. \$\endgroup\$ Commented Apr 7, 2020 at 19:31
  • \$\begingroup\$ @RonBeyer the design doesnt specify the flash device sadly, and yes i am aware of impedence, length matching and even signal synchronization (if thats what it is called in english) and looping. the package is TSOP. which is manageable by prototyping service i am using. and if you use a similar device in your project, just tell me how u approach this pins. \$\endgroup\$ Commented Apr 7, 2020 at 19:37
  • \$\begingroup\$ In my project, these pins are tied directly to the NAND flash controller in my MPU, which manages the pull/ups required for operation. They must be connected because the Micron flash memory uses the WE/RE to latch in commands (see page 23 of your datasheet). I don't know (you don't say) how you are interfacing USB to this NAND flash chip, so I'm not sure what to tell you to do with them... \$\endgroup\$ Commented Apr 7, 2020 at 19:43

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In general, no signal should be left floating, but in reality, external pull-ups and pull-downs can be left off from non-critical signals.

Usually the assumption is that signals do not float for too long, because MCU starts executing firmware and will soon initialize the IO pins. It may be longer without initializing the IO pins during initial boot when it has no firmware, or during further firmware updates.

If a critical line, such as Write Protect, is correctly set with external resistors, the Flash chip can see floating garbage on all other pins, but would not write anything.

Or if the Chip Select is correctly set with external resistors, the Flash chip can have floating garbage on all other pins, but it won't listen to any of it because chip is properly disabled.

It really depends on the chip how important those external resistors are.

I prefer to put pull resistors on almost all important control pins, just because it is better to have a place for it and not mount it, than not to have a place for it and find out it is necessary.

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