From: Luke Kenneth Casson Leighton <lkcl@lkcl.net>
Date: Mon, 28 Feb 2022 17:43:39 +0000 (+0000)
Subject: attempting to introduce an extra few clock cycles delay on power-up
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=cc9d98a1dcc6c307c4a716fab0bbddb11ac9be39;p=soc.git

attempting to introduce an extra few clock cycles delay on power-up
this may help with initialisation of I-Cache SRAM which is combinatorial.
maybe
---

diff --git a/src/soc/simple/issuer.py b/src/soc/simple/issuer.py
index 379c373e..cfad831a 100644
--- a/src/soc/simple/issuer.py
+++ b/src/soc/simple/issuer.py
@@ -436,10 +436,14 @@ class TestIssuerBase(Elaboratable):
             dbg_sync = ClockDomain(self.dbg_domain)
             m.domains += dbg_sync
 
+        # create a delay, but remember it is in the power-on-reset clock domain!
         ti_rst = Signal(reset_less=True)
         delay = Signal(range(4), reset=3)
+        stop_delay = Signal(range(16), reset=5)
         with m.If(delay != 0):
-            m.d.por += delay.eq(delay - 1)
+            m.d.por += delay.eq(delay - 1) # decrement... in POR domain!
+        with m.If(stop_delay != 0):
+            m.d.por += stop_delay.eq(stop_delay - 1) # likewise
         comb += cd_por.clk.eq(ClockSignal())
 
         # power-on reset delay
@@ -450,6 +454,9 @@ class TestIssuerBase(Elaboratable):
         else:
             with m.If(delay != 0 | dbg.core_rst_o):
                 comb += core_rst.eq(1)
+        with m.If(stop_delay != 0):
+            # run DMI core-stop as well but on an extra couple of cycles
+            comb += dbg.core_stopped_i.eq(1)
 
         # connect external reset signal to DMI Reset
         if self.dbg_domain != "sync":
@@ -815,6 +822,12 @@ class TestIssuerInternal(TestIssuerBase):
 
         with m.FSM(name='fetch_fsm'):
 
+            # allow fetch to not run at startup due to I-Cache reset not
+            # having time to settle.  power-on-reset holds dbg.core_stopped_i
+            with m.State("PRE_IDLE"):
+                with m.If(~dbg.core_stopped_i & ~dbg.core_stop_o):
+                    m.next = "IDLE"
+
             # waiting (zzz)
             with m.State("IDLE"):
                 # fetch allowed if not failed and stopped but not stepping