-Access without symmetry was awkward, now we can reach regs from TX or RX side
-Removes a special path for mailbox (came for free)
-At the same time reduced clock complexity (one clock for system!!)
-Moved mailbox to top level
-Changed main clock to "sys_clk" for all
-A read on transmit side initiates timer
-Timer is reset when read request comes back
(assumption last read requests starts timer..last return stops it)
