2687 - MachXO2: What is best practice for the MachXO2 Slave SPI Chip Select (SN)?
The Slave SPI (SSPI) Chip Select (SN) input signal is recommended to be pulled high using an external pull-up resistor.
The SSPI configuration port is the second highest in boot priority, with JTAG being the highest priority. Assertion of the SN input, deliberate or not, can cause the MachXO2 to fail to program, and to configure. Even if you are not planning to use the SSPI configuration port it is recommended the SN pin be pulled high.
The SSPI configuration port is active when the MachXO2 is in Default Mode Feature Row state. Default Mode Feature Row state occurs when the Configuration Flash, User Flash Memory, Feature Row, and SRAM cells are all erased. In this state the SSPI configuration port is enabled. Assertion of SN in this state may prevent programming and configuration of the MachXO2 from the I2C and Wishbone configuration ports.
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