drtio: add Xilinx RX synchronizer

artiq/gateware/drtio/xilinx_rx_synchronizer.py

@@ -0,0 +1,38 @@
+from migen import *
+
+
+class XilinxRXSynchronizer(Module):
+    """Deterministic RX synchronizer using a relatively placed macro
+    to put the clock-domain-crossing FFs right next to each other.
+
+    To meet setup/hold constraints receiving FFs, adjust the phase shift 
+    of the Si5324. 
+
+    We assume that FPGA routing variations are small enough to be negligible.
+    """
+    def __init__(self):
+        self.signals = []
+
+    def resync(self, signal):
+        synchronized = Signal.like(signal, related=signal)
+        self.signals.append((signal, synchronized))
+        return synchronized
+
+    def do_finalize(self):
+        l = sum(len(s[0]) for s in self.signals)
+        din = Signal(l)
+        inter = Signal(l)
+        dout = Signal(l)
+        self.comb += [
+            din.eq(Cat(*[s[0] for s in self.signals])),
+            Cat(*[s[1] for s in self.signals]).eq(dout)
+        ]
+
+        for i in range(l):
+            hu_set = ("HU_SET", "drtio_rx_synchronizer")
+            self.specials += [
+                Instance("FD", i_C=ClockSignal("rtio_rx"), i_D=din[i], o_Q=inter[i],
+                         attr={hu_set, ("RLOC", "X0Y{}".format(i))}),
+                Instance("FD", i_C=ClockSignal("rtio"), i_D=inter[i], o_Q=dout[i],
+                         attr={hu_set, ("RLOC", "X1Y{}".format(i))})
+            ]