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18.104.22.168. Check Read-During-Write Behavior
Your HDL source code specifies the memory behavior when you read and write from the same memory address in the same clock cycle. The read returns either the old data at the address, or the new data written to the address. This is referred to as the read-during-write behavior of the memory block. Intel FPGA memory blocks have different read-during-write behavior depending on the target device family, memory mode, and block type.
Synthesis tools preserve the functionality described in your source code. Therefore, if your source code specifies unsupported read-during-write behavior for the RAM blocks, the Intel® Quartus® Prime software implements the logic in regular logic cells as opposed to the dedicated RAM hardware.
Continuous read in HDL code
One common problem occurs when there is a continuous read in the HDL code, as in the following examples. Avoid using these coding styles:
//Verilog HDL concurrent signal assignment assign q = ram[raddr_reg];
-- VHDL concurrent signal assignment q <= ram(raddr_reg);
This type of HDL implies that when a write operation takes place, the read immediately reflects the new data at the address independent of the read clock, which is the behavior of asynchronous memory blocks. Synthesis cannot directly map this behavior to a synchronous memory block. If the write clock and read clock are the same, synthesis can infer memory blocks and add extra bypass logic so that the device behavior matches the HDL behavior. If the write and read clocks are different, synthesis cannot reliably add bypass logic, so it implements the logic in regular logic cells instead of dedicated RAM blocks. The examples in the following sections discuss some of these differences for read-during-write conditions.
In addition, the MLAB memories in certain device logic array blocks (LABs) does not easily support old data or new data behavior for a read-during-write in the dedicated device architecture. Implementing the extra logic to support this behavior significantly reduces timing performance through the memory.
In many synthesis tools, you can declare that the read-during-write behavior is not important to your design (for example, if you never read from the same address to which you write in the same clock cycle). In Intel® Quartus® Prime Pro Edition synthesis, set the synthesis attribute ramstyle to no_rw_check to allow Intel® Quartus® Prime software to define the read-during-write behavior of a RAM, rather than use the behavior specified by your HDL code. This attribute can prevent the synthesis tool from using extra logic to implement the memory block, or can allow memory inference when it would otherwise be impossible.
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