Update on Serial-interface NVM Expansion Board


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A while back I posted about these boards being put in for order through OSH Park.  Well, I am happy to say the rev1 boards came in today; however, with much to my dismay I am afraid there is a fatal design flaw with the SPI configurations of this board…  Nothing that would render this first batch *completely* useless, but in my mind this defect might as well make them useless.  Maybe it’s just my expectation of perfection, especially with fairly simple designs as this.  As do most people, I also make design errors from time to time, but I do take pride in getting things right the first time, which this time I did not – waaa waaaa 🙁  What I realized as I was preparing to solder components onto the first SPI flash board was this: in order to stop the write process to the NVM, the CS line must be released.  In order to release the CS line to the NVM chip, two bytes must be sent to the MCP23S08 chip to de-assert the output line to that NVM chip.  Well, the NVM chip will receive these two bytes and commit them to memory in the process of de-asserting the NVM chip CS line, as this output from the MCP23S08 will still be asserted until latching the 8th bit of the second byte and the MCU releases the CS line to the MCP23S08.  Again, waaa waaaa 🙁  Looks like I’ll be sending Laen @OSH Park more business in the near future…

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