- 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.
- reduced frame fanout, removed clock gater in erx_io (improves speed path)
- driving constants on "wid signals" (proper)
- making lock signal 1 bit wide to remove warning
- moved backed to BUFIO for IDDR blocks
- Giving a wait on every ack just doesn't make sense on the read port with a fifo there??
- Makes for a nasty combinatorial loop during integration.
- Test passes (but need to look into this more)
-Apparantly old FIFO was not pipelined (IE data comes back same cycle).
-Not knowing the Xilinx logic, I made it a regular one cycle pipeline
memory based FIFO
-First bug was a typo. Cursing AXI for making every signal look exactly the same at first glance. Not good use practice
-Second bug was sloppy. (removed pipeline stage on write data by mistake)
-added register read/write properly
-removed redundant wrapper layers in maxi/saxi
-changed over to "emesh" interface from packet 103 bit data
-cleaned up maxi
-cleaned up saxi
-removed redundant signals in elink interface (user,lock,..)
-added wrapper to fifo (to carry emesh interface through)
Now comes the fun part of testing
Our "standard packet" order should be followed everywhere to ease verification and integration (standards are good fir reuse...):
[0]=access
[1]=write
[3:2]=datamode
[7:4]=ctrlmode
[39:8]=dstaddr
[71:40]=data
[103:72]=upper-data (or srcaddr)
