QSPI Flash Chip Select Drive Strength

Hi,

Just found out there is a 240 ohm pull-up resistor on the QSPI_1_CS# (QSPI flash chip select) in MicroZed schematic and the PS_MIO1 needs to sink >13mA to drive the QSPI_1_CS# to 0V. But PS MIO is rated 8mA in the datasheet. Is there a special reason to pull-up the QSPI_1_CS# with a 240 ohm?

Thanks!

0 bhfletcher over 10 years ago

I'm looking into this now. I see on other Zynq boards (ZedBoard, ZC702, ZC706) that a 330-ohm was used, but even that seems like overkill compared to what the Spansion datasheet says for this signal.

A simple pull-up resistor (generally of the order of 100 k-ohm) on Chip Select (CS#) can usually be used to insure safe and proper power-up and power-down.

Seems that 240-ohm is definitely not necessary, and we can definitely change this for future builds. I'm checking with Xilinx to see why they selected 330-ohm, and I'll report back.

Bryan
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0 bhfletcher over 10 years ago

Out of curiosity, were you seeing a failure? With all of our testing, including extended testing and temperature in a chamber, we have not seen any failures.
Bryan
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0 Former Member over 10 years ago in reply to bhfletcher

Hi Bryan,

Thanks for the help. No Failure found. Just a question found during a schematic check.

Chao
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0 bhfletcher over 10 years ago

From what I can tell, Xilinx didn't have a specific reason for choosing 330-ohms on their ZC702/706 pull-up. I suspect something >1K would be a better choice. I have asked our engineering group to test a higher value.
Bryan
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0 bhfletcher over 10 years ago
We measured R71’s low signal level with the following resistors:

R71 at 240 yields an output low(max) of 200mV.

R71 at 100K yields an output low (max) of 60mV.

These values were captured using a DMM. By inference, we can get I by: (3.3V-0.2V)/R71 = 3.1V/240 = 12.9mA. At 100K we see 32.4uA.

We are planning to increase R71 for our next MicroZed build. Thanks for bringing this to our attention.

Bryan
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