Electronic – FPGA Configuration Flash as General Purpose Memory After Boot

I'm not sure what you mean.

When you store 0xFF, it stores 0xFF - that is, it sets all 8 bits of the register. The exact same thing happens when you erase the chip. Look at erasing as bulk writing 0xFF to all registers.

In both cases, a read operation would return 0xFF.

I think from your question you already understand this, but for clarification, this is my usual terminology:

• Programming implies altering some non-volatile device
• Configuration is what the FPGA does when it starts up - it loads its volatile internal memory from non-volatile external memory.

So, as you surmise, the system needs a non-volatile device of some sort which you program on the production line. To Clarify your comment on the USB blaster, it is not a chip... it's a "pod", so you'd have one for your programming station, not one per board:

Anyway, back to the non-volatile storage options:

• The "Almost-zero-engineering required" approach uses Altera's configuration flash devices, which are JTAG programmable using something like a USB blaster. You just wire them up like the datasheet says, and away you go. They cost more per MBit than other options, though, so are not often used in volume production.

• One more usual approach is a "normal" SPI flash chip. The Cyclone user guide configuration section will list some which are compatible with it, and I believe you can also use the Quartus software to program them via the JTAG of the FPGA they are connected to. More engineering involved, checking you have the right device etc. Also, if you are in volume production, you may not want to be using Quartus on the production line, which which case you may have to provide a separate programming header for the flash chip, and some hardware+software to drive that.

• If you have a microcontroller in your system (even on another board...) you could connect that up to the FPGA's JTAG or configuration pins and store the FPGA bitstream in the micro's flash. More engineering involved as you have to have some software to "boot" the FPGA. however, it can make in-field upgrades easier, as often the micro is set up to receive software upgrades already, whereas updating the flash when it hangs directly off the FPGA is often an "opening the box" experience!