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Discussion - Hardware / Re: FT601Q: 245 Synchronous FIFO mode. TXE_N indicator width.
« on: October 22, 2019, 10:19:05 am »Not only. USB side of FTDI sends data to PC serially, not parallely.
Perhaps due to unaligned read/write.
So even when data is written to FTDI aligned (32 bit), it may disappears from FIFO byte by byte.
When I write alligned 1024 words of 32 bits, I fill all the FIFO. But FIFO will be send
via USB byte by byte, right?
It depends on what FIFO occupancy meter is used.
TXE_N indicates 1 byte, and nonaligned writes are supported, so I guess that meter is 1 byte, and counts to 4095. Every 32-bit (aligned) write to FTDI increase FIFO occupancy by 4. But continuous transmission via USB will decrease FIFO occupancy by 1 or by 4. It depends on FTDI hardware, which is not described in datasheet.
If it decreased by 1, it will cause issue that I asking for. Even when I write only aligned 32-bit data, FTDI
may sends it byte by byte indicating 1 byte free space in FIFO.
In this situation TXE_N
is useless, because it don't give enough information to next 32-bit aligned write. To do 32-bit writes, a 4-bytes free space information is necessary.
But in Your scenario, when nonaligned writes are made, the same issue appear even when USB transmission decreasing FIFO occupancy by 4.
So I'm surprised that You made TXE_N as 1-byte free space indicator, not as 4-byte free space indicator.
Unless it's a datasheet error and it's actually a 4-byte indicator. So I ask about that.