Xilinx SPI flash programming with MCU

Sometimes we wish we could program the SPI flash connected to Xilinx FPGA using some MCU. This is easy if the SPI flash pins are accessible. But what can we do if they are not?

Option 1:

If we want we still can use an external MCU for this we need to add "SPI bypass" wires into all designs. SPI flash pins are accessible for the design, we can just wire them to the pins that go to the MCU. We need to use STARTUP directive to get access to the SPI clock, other pins are simply assignable in the constraints.

We have to use JTAG tools once to program the SPI flash with the design that includes the SPI bypass. And later we need to be sure only program images that include the bypass logic. To make it failsafe we can also write a golden image to the end of the SPI flash that we never erase, so if the first image is corrupt the FPGA would still configure with the golden image.

Option 2: We connect the SPI flash to FPGA embedded processor (MicroBlaze) and implement the SPI flash update with Microblaze code, while the external MCU simply gives the code bytes to program into the flash.