pipistrello: add notes to nist_qc1 about dds_clock

* remove xtrig from the target as it is not usually connected (used for
  dds_clock) and ignore PMT2/BTN2 as C:15 is used for dds_clock.
* this also aligns the ttl channel numbers with kc705/nist_qc1 (two pmt
  inputs followed by 16 ttl outputs followed by leds)

artiq/gateware/nist_qc1.py

@@ -4,10 +4,22 @@ from mibuild.generic_platform import *
 papilio_adapter_io = [
     ("ext_led", 0, Pins("B:7"), IOStandard("LVTTL")),
 
+    # to feed the 125 MHz clock (preferrably from DDS SYNC_CLK)
+    # to the FPGA, use the xtrig pair.
+    #
+    # on papiliopro-adapter, xtrig (C:12) is connected to a GCLK
+    #
+    # on pipistrello, C:15 is the only GCLK in proximity, used as a button
+    # input, BTN2/PMT2 in papiliopro-adapter
+    # either improve the DDS box to feed 125MHz into the PMT2 pair, or:
+    #
+    # * disconnect C:15 from its periphery on the adapter board
+    # * bridge C:15 to the xtrig output of the transciever
+    # * optionally, disconnect C:12 from its periphery
+    ("xtrig", 0, Pins("C:12"), IOStandard("LVTTL")),
     ("pmt", 0, Pins("C:13"), IOStandard("LVTTL")),
     ("pmt", 1, Pins("C:14"), IOStandard("LVTTL")),
-    ("xtrig", 0, Pins("C:12"), IOStandard("LVTTL")),
+    ("pmt", 2, Pins("C:15"), IOStandard("LVTTL")),  # rarely equipped
-    ("dds_clock", 0, Pins("C:15"), IOStandard("LVTTL")),  # PMT2
 
     ("ttl", 0, Pins("C:11"), IOStandard("LVTTL")),
     ("ttl", 1, Pins("C:10"), IOStandard("LVTTL")),

soc/targets/artiq_pipistrello.py

@@ -34,7 +34,7 @@ class _RTIOCRG(Module, AutoCSR):
                                   i_FREEZEDCM=0,
                                   i_RST=ResetSignal())
 
-        rtio_external_clk = platform.request("dds_clock")
+        rtio_external_clk = platform.request("pmt", 2)
         platform.add_period_constraint(rtio_external_clk, 8.0)
         self.specials += Instance("BUFGMUX",
                                   i_I0=rtio_internal_clk,
@@ -95,11 +95,13 @@ trce -v 12 -fastpaths -tsi {build_name}.tsi -o {build_name}.twr {build_name}.ncd
         for i in range(2):
             phy = ttl_simple.Inout(platform.request("pmt", i))
             self.submodules += phy
-            rtio_channels.append(rtio.Channel.from_phy(phy, ififo_depth=512))
+            rtio_channels.append(rtio.Channel.from_phy(phy, ififo_depth=512,
+                                                       ofifo_depth=4))
 
-        phy = ttl_simple.Inout(platform.request("xtrig", 0))
+        phy = ttl_simple.Inout(platform.request("xtrig"))
         self.submodules += phy
-        rtio_channels.append(rtio.Channel.from_phy(phy, ififo_depth=4))
+        rtio_channels.append(rtio.Channel.from_phy(phy, ififo_depth=4,
+                                                   ofifo_depth=4))
 
         for i in range(16):
             phy = ttl_simple.Output(platform.request("ttl", i))