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There are very often two ways to use memory on modern FPGAs. Either there are memory blocks on board the FPGA (or on the development board) or you wish to make your own memory blocks for storage using the flip flops on the FPGA logic. Either way, it is important to realize that there are significant differences between dedicated high density Dynamic RAM (DRAM) and Synchronous Dynamic Random Access Memory (SDRAM) and flip-flop based Static RAM (SRAM).
If we consider SDRAM, the key aspects of this type of memory to consider are:
This type of DRAM relies on transistor capacitance on gates to store data.
DRAM is much more compact than SRAM (Static RAM).
DRAM cannot be synthesized; you need a separate DRAM chip.
SDRAM requires a synchronization clock that is consistent with the rest of the hardware system (it is designed to operate with microprocessors).
DRAM data must be refreshed as it is stored charge and decays after a certain time.
DRAM is slower than SRAM.
Static RAM can be considered in a similar way to a ROM chip and it also has (differing) key aspects of behavior to consider:
Memory cells are based on standard latches.
SRAM is fast.
SRAM is less compact than DRAM (or SDRAM).
SRAM can be synthesized on an FPGA so is ideal for small, fast registers or memory blocks.
Static RAM is essentially asynchronous, but can be modified to behave synchronously (as SDRAM is the synchronous equivalent of DRAM), and this is often called Synchronous RAM.
Flash memory is useful to consider at this point, even though its operation is fundamentally different from the memory types considered thus far, simply because it is easy to use and is commonly available on many FPGA development boards. Flash memory is essentially a form of EEPROM (electrically programmable ROM) that can be used as a form of persistent RAM. Why persistent? In flash memory, the device memory is retained even when the power is removed, so it is often used as a form of ROM, which makes it an interesting memory to use on FPGA systems as it could be used to store the FPGA program, but also used as a RAM storage (dynamically) for current data.
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