- After finding the bug in the reference model and wasting countless hours going back and forth with FPGA timing optimization and bug tweaks, I realized that the design was fundementally broken. The decision to use two clock domains (high speed) and low speed was correct from the beginning. The FPGA is dreadfully slow, (you definitely don't want to do much logic at 300MHz...), but the handoff between tclk and tclk_div4 was too complicated. The puzzle of having to respond to wait quickly, covering the corner cases, and meeting timing was just too ugly.
- The "new" design goes back to the method of using the high speed logic only for doing a "dumb" parallel to serial converter and preparing all the necessary signals in the low speed domain.
- This feel A LOT cleaner and the it already passes basic tests with the chip reference and the loopback after less than 3 hours of redesign work!
- The TX meets timing but there is still some work to do with wait pushback testing.
- Turns out I had a nasty bug that was masked by using my own RX to loopback the TX. Since the new RX is very benign with a programmable fifo full flag the timing is quite relaxed.
- The legacy elink for e16 has a strict wait policy. When wait is raised high, you must stop pretty much immediately.
- I struggled with testing this bug on the parallella for 2 days.
- Putting together the test environment uncovered the bug in a couple of hours. F**K, I should know better!!
- The axi slave can never drive enough reads to saturate the maxi fifo since it's only sending out one read at a time.
- Changing the system so that a raw elink sits in front of stimulus..
-Need to get into this again! (don't like this part of code still..)
-One lesson, if you are unsure of something leave the old code in comment...can save a lot of time.
- Turns out there was a bug hidden in the emaxi that can only be found by properly driving a master device with reads. This could not happen in the old environment.
- Note that due to limitations in the esaxi, I had to add the etx_fifo block as an interface (simplest).
- The ESAXI is very limited in that it MUST interface to a fifo with spare entries. (so prog_full). This should be FIXED!
- Minimal test passes, now to try to reproduce the DMA bug..
- Clearing the "done" register with tx_burst. Kind of makes sense logically since while we are in burst mode we are not done.
- Still not 100% happy with this circuit, but there arent' a lot of lines of code left...
- But elink now passes 500 random burst transactions!!!
- Adding transaction counter to speed up debugging
- Clearing access signal on wait ("bubble")
- Adding back special propagation when there is a wait after io_wait.
- This is a pain in the ass and should never have been implemented in the first place!
- Burst information is contained in two places, once in the first byte being transmitted and once by the frame staying high
- This was done because there was a second special bursting mode where data is streamed into the same address, so bit[2] becomes a "command bit".
-Solved a speed path in synchronizing the wait signal, had to use the first edge signal fo the IO and the lclk_div4 for the core logic. It seems that the FPGA has a really hard time mixing clock domains, the routing delay between domains explodes
-Put in some special case logic for edge cases, like when there is a wait coming in from the IO and there is a wait from the IO. In that case, the packet gets sampled by the IO and not by the current logic.
-This needs to be cleaned up eventually, not clean enough but it's good enough for now.
- The burst signal was going fro lclk_div4 domain straight into the io high speed domain. There is quite a bit of logic on this signal. Instead of starting with false paths or multi cycle paths with firstedge, I changed the pipeline.
- The logic was a mess, causing me to go around in circles for days. In the end, by adding a missing sync circuit (duh!) between the fast and slow clock to align the edges and removing a redundant pipeline stage ("double") the nasty logic just fell away. Looks good now.
-Write bursts mostly works and design looks clean.
-one bug left to fix on streams of writes...
- The burst signal needs to be pipelined like everything else (0th order..)
- Don't look at write signal when pushing back wait...WILL GO BACK AND REVISIT THIS ONE LATER.
- Yeah, burst write test now passes!!!!
- Old design was not workable with bursting and long waits. The wait signal needs to be very carfully handled since it's asynchronous to the clock.
-The TX needs to be stopped quickly so the sync needs to be done at the high speed clock, not at div4 clock
-Since there are synchronizers here, there should be only one point of sync. This is not completely the case still, but I think??? it should be safe by constructiona at this point.
-bursting working at this point for writes!!!!!
