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Added mem_wstrb documentation

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Clifford Wolf 2016-04-03 17:14:07 +02:00
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@ -319,6 +319,11 @@ In a write transfer `mem_wstrb` is not 0 and `mem_rdata` is unused. The memory
write the data at `mem_wdata` to the address `mem_addr` and acknowledges the write the data at `mem_wdata` to the address `mem_addr` and acknowledges the
transfer by asserting `mem_ready`. transfer by asserting `mem_ready`.
The 4 bits of `mem_wstrb` are write enables for the four bytes in the addressed
word. Only the 8 values `0000`, `1111`, `1100`, `0011`, `1000`, `0100`, `0010`,
and `0001` are possible, i.e. no write, write 32 bits, write upper 16 bits,
write lower 16, or write a single byte respectively.
There is no need for an external wait cycle. The memory can acknowledge the There is no need for an external wait cycle. The memory can acknowledge the
write immediately with `mem_ready` going high in the same cycle as write immediately with `mem_ready` going high in the same cycle as
`mem_valid`, or `mem_ready` being tied to constant 1. `mem_valid`, or `mem_ready` being tied to constant 1.