From b52f253dbdc3bf80eb0aa6343f018c2527da683e Mon Sep 17 00:00:00 2001
From: occheung <54844539+occheung@users.noreply.github.com>
Date: Mon, 25 Sep 2023 17:02:49 -0700
Subject: [PATCH] Simplify OOB reset by clock division (#2217)

* oob: simply logic by dividing into clk100

* replace clk100 clk ctrl with clk200 async reset

* fix comment (singular/plural)

* oob reset: invoke platform commands locally

* cleanup

* oob reset: add async reset import

* fix duplicated comment
---
 .../gateware/drtio/transceiver/eem_serdes.py  | 104 +++++++++++-------
 1 file changed, 65 insertions(+), 39 deletions(-)

diff --git a/artiq/gateware/drtio/transceiver/eem_serdes.py b/artiq/gateware/drtio/transceiver/eem_serdes.py
index 4092cb202..75b4f3389 100644
--- a/artiq/gateware/drtio/transceiver/eem_serdes.py
+++ b/artiq/gateware/drtio/transceiver/eem_serdes.py
@@ -1,4 +1,5 @@
 from migen import *
+from migen.genlib.resetsync import AsyncResetSynchronizer
 from misoc.interconnect.csr import *
 from misoc.cores.code_8b10b import SingleEncoder, Decoder
 from artiq.gateware.drtio.core import TransceiverInterface, ChannelInterface
@@ -396,53 +397,78 @@ class SerdesSingle(Module):
 
 
 class OOBReset(Module):
-    def __init__(self, iserdes_o):
-        ce_counter = Signal(13)
-        activity_ce = Signal()
-        transition_ce = Signal()
+    def __init__(self, platform, iserdes_o):
+        self.clock_domains.cd_clk100 = ClockDomain()
+        self.specials += [
+            Instance("BUFR",
+                i_I=ClockSignal("clk200"),
+                o_O=ClockSignal("clk100"),
+                p_BUFR_DIVIDE="2"),
+            AsyncResetSynchronizer(self.cd_clk100, ResetSignal("clk200")),
+        ]
 
-        self.sync.clk200 += Cat(ce_counter, activity_ce).eq(ce_counter + 1)
-        self.comb += transition_ce.eq(ce_counter[0])
-
-        idle_low_meta = Signal()
-        idle_high_meta = Signal()
         idle_low = Signal()
         idle_high = Signal()
 
-        idle = Signal()
         self.rst = Signal(reset=1)
 
-        # Detect the lack of transitions (idle) within 2 clk200 cycles
-        self.specials += [
-            Instance("FDCE", p_INIT=1, i_D=1, i_CLR=iserdes_o,
-                i_CE=transition_ce, i_C=ClockSignal("clk200"), o_Q=idle_low_meta,
-                attr={"async_reg", "ars_ff1"}),
-            Instance("FDCE", p_INIT=1, i_D=idle_low_meta, i_CLR=0,
-                i_CE=transition_ce, i_C=ClockSignal("clk200"), o_Q=idle_low,
-                attr={"async_reg", "ars_ff2"}),
+        # Detect the lack of transitions (idle) within a clk100 cycle
+        for idle, source in [
+                (idle_low, iserdes_o), (idle_high, ~iserdes_o)]:
+            idle_meta = Signal()
+            ff_pair = [ff1, ff2] = [
+                Instance("FDCE", p_INIT=1, i_D=1, i_CLR=source,
+                    i_CE=1, i_C=ClockSignal("clk100"), o_Q=idle_meta,
+                    attr={"async_reg"}),
+                Instance("FDCE", p_INIT=1, i_D=idle_meta, i_CLR=0,
+                    i_CE=1, i_C=ClockSignal("clk100"), o_Q=idle,
+                    attr={"async_reg"}),
+            ]
+            self.specials += ff_pair
 
-            Instance("FDCE", p_INIT=1, i_D=1, i_CLR=~iserdes_o,
-                i_CE=transition_ce, i_C=ClockSignal("clk200"), o_Q=idle_high_meta,
-                attr={"async_reg", "ars_ff1"}),
-            Instance("FDCE", p_INIT=1, i_D=idle_high_meta, i_CLR=0,
-                i_CE=transition_ce, i_C=ClockSignal("clk200"), o_Q=idle_high,
-                attr={"async_reg", "ars_ff2"}),
-        ]
+            platform.add_platform_command(
+                "set_false_path -quiet -to {ff1}/CLR", ff1=ff1)
+            # Capture transition detected by FF1/Q in FF2/D
+            platform.add_platform_command(
+                "set_max_delay 2 -quiet "
+                "-from {ff1}/Q -to {ff2}/D", ff1=ff1, ff2=ff2)
 
-        # Detect activity for the last 2**13 clk200 cycles
-        # The 2**13 cycles are fully partitioned into 2**12 time segments of 2
-        # cycles in duration. If there exists 2-cycle time segment without
-        # signal level transition, rst is asserted.
-        self.sync.clk200 += [
-            If(activity_ce,
-                idle.eq(0),
-                self.rst.eq(idle),
-            ),
+        # Detect activity for the last 2**15 clk100 cycles
+        self.submodules.fsm = fsm = ClockDomainsRenamer("clk100")(
+            FSM(reset_state="WAIT_TRANSITION"))
+        counter = Signal(15, reset=0x7FFF)
+
+        # Keep sysclk reset asserted until transition is detected for a
+        # continuous 2**15 clk100 cycles
+        fsm.act("WAIT_TRANSITION",
+            self.rst.eq(1),
             If(idle_low | idle_high,
-                idle.eq(1),
-                self.rst.eq(1),
-            ),
-        ]
+                NextValue(counter, 0x7FFF),
+            ).Else(
+                If(counter == 0,
+                    NextState("WAIT_NO_TRANSITION"),
+                    NextValue(counter, 0x7FFF),
+                ).Else(
+                    NextValue(counter, counter - 1),
+                )
+            )
+        )
+
+        # Reassert sysclk reset if there are no transition for the last 2**15
+        # clk100 cycles.
+        fsm.act("WAIT_NO_TRANSITION",
+            self.rst.eq(0),
+            If(idle_low | idle_high,
+                If(counter == 0,
+                    NextState("WAIT_TRANSITION"),
+                    NextValue(counter, 0x7FFF),
+                ).Else(
+                    NextValue(counter, counter - 1),
+                )
+            ).Else(
+                NextValue(counter, 0x7FFF),
+            )
+        )
 
 
 class EEMSerdes(Module, TransceiverInterface, AutoCSR):
@@ -526,7 +552,7 @@ class EEMSerdes(Module, TransceiverInterface, AutoCSR):
 
         self.submodules += serdes_list
 
-        self.submodules.oob_reset = OOBReset(serdes_list[0].rx_serdes.o[0])
+        self.submodules.oob_reset = OOBReset(platform, serdes_list[0].rx_serdes.o[0])
         self.rst = self.oob_reset.rst
         self.rst.attr.add("no_retiming")