[SOLVED] Selecting an FPGA PROM

[ThePilot]

Hello,
I'm teaching myself how to design using FPGAs and VHDL. I'm at a stage where I'm putting a small PCB together to test out some ideas.

The only thing I'm not sure about is how to select a flash chip/PROM/config chip to store the code. I see on websites like Digikey and Farnell that thousands exist - how do I pick one!?






Thanks :!:

 

[alexan_e]

I assume you mean an eeprom or flash because a PROM can only be programmed once.
Have you checked the datasheet of your FPGA, there should be a few recommendations there.

Alex

 

[ThePilot]

Thanks, I meant is there anything specific that I should look for? I'm especially concerned about the size it needs to be. What is the maximum the FPGA will need?

The reason I ask is that normal code (program, C, ASM etc) is a series of instructions that a processor will crunch through. The program can be as long as it needs to be, and the processor will keep going until it reaches the end. An FPGA is different in that it is constrained by a number of gates, logic blocks and memories. If I were to use every single feature of an FPGA, how would I work out how big the flash would need to be? Or, to think of it another way, if I'm doing VHDL and hardware design in parallel, how do I ensure the flash will be large enough?



The datasheet just says what is printed below. I'll probably use a SPI flash because they are nice and cheap.

Master Serial from a Xilinx Platform Flash PROM
Serial Peripheral Interface (SPI) from an
industry-standard SPI serial Flash
Byte Peripheral Interface (BPI) Up from an
industry-standard x8 or x8/x16 parallel NOR Flash
Slave Serial, typically downloaded from a processor
Slave Parallel, typically downloaded from a processor
Boundary Scan (JTAG), typically downloaded from a
processor or system tester

Thanks!

 

[alexan_e]

The size needed depends on the FPGA size so there should be a table in your datasheet listing the size of each model.
Which specific FPGA model do you intend to use?

 

[ThePilot]

I'm looking at the datasheet for the Xilinx Spartan-3A range, the XC3S50A in particular.

---------- Post added at 01:08 ---------- Previous post was at 01:03 ----------

hmmm, I've found a different datasheet with a table headed "Uncompressed bitstream image" which says "437,312" for the device I'm looking at. Is this what I've been looking for? I'll need 0.5Mbit flash, right?

 

[alexan_e]

If you check https://www.xilinx.com/support/documentation/user_guides/ug332.pdf there is a table in page 39 with the number of configuration bits on each device, XC3S50A/AN has 437312 bits.