VCD: Add samplerate support to fix displayed timestamps

To use this, pass the samplerate kwarg to LiteScopeAnalyzer(). If using the sys domain, soc_obj.sys_clk_freq works.

examples/arty.py

@@ -48,6 +48,7 @@ class LiteScopeSoC(BaseSoC):
         self.submodules.analyzer = LiteScopeAnalyzer(analyzer_signals,
             depth        = 1024,
             clock_domain = "sys",
+            samplerate   = self.sys_clk_freq,
             csr_csv      = "analyzer.csv")
         self.add_csr("analyzer")
 

litescope/core.py

@@ -226,9 +226,10 @@ class _Storage(Module, AutoCSR):
 
 
 class LiteScopeAnalyzer(Module, AutoCSR):
-    def __init__(self, groups, depth, clock_domain="sys", trigger_depth=16, register=False, csr_csv="analyzer.csv"):
+    def __init__(self, groups, depth, samplerate=1e-12, clock_domain="sys", trigger_depth=16, register=False, csr_csv="analyzer.csv"):
         self.groups     = groups = self.format_groups(groups)
         self.depth      = depth
+        self.samplerate = samplerate
 
         self.data_width = data_width = max([sum([len(s) for s in g]) for g in groups.values()])
 
@@ -294,6 +295,7 @@ class LiteScopeAnalyzer(Module, AutoCSR):
             return ",".join(args) + "\n"
         r = format_line("config", "None", "data_width", str(self.data_width))
         r += format_line("config", "None", "depth", str(self.depth))
+        r += format_line("config", "None", "samplerate", str(self.samplerate))
         for i, signals in self.groups.items():
             for s in signals:
                 r += format_line("signal", str(i), vns.get_name(s), str(len(s)))

litescope/software/driver/analyzer.py

@@ -118,6 +118,7 @@ class LiteScopeAnalyzerDriver:
         self.add_trigger(value, mask, cond)
 
     def configure_subsampler(self, value):
+        self.subsampling = value
         self.subsampler_value.write(value-1)
 
     def run(self, offset=0, length=None):
@@ -166,11 +167,13 @@ class LiteScopeAnalyzerDriver:
         return self.data
 
     def save(self, filename, samplerate=None, flatten=False):
+        if samplerate is None:
+            samplerate = self.samplerate / self.subsampling
         if self.debug:
             print("[writing to " + filename + "]...")
         name, ext = os.path.splitext(filename)
         if ext == ".vcd":
-            dump = VCDDump()
+            dump = VCDDump(samplerate=samplerate)
         elif ext == ".csv":
             dump = CSVDump()
         elif ext == ".py":

litescope/software/dump/vcd.py

@@ -6,6 +6,7 @@
 
 from itertools import count
 import datetime
+import re
 from litescope.software.dump.common import Dump, dec2bin
 
 
@@ -19,14 +20,37 @@ def vcd_codes():
             code = codechars[r] + code
         yield code
 
+_si_prefix2exp = {
+    "":    0,
+    "m":  -3,
+    "u":  -6,
+    "n":  -9,
+    "p": -12,
+    "f": -15,
+}
+
+def _timescale_str2num(timescale):
+    match = re.fullmatch("(\d+)(\w{0,1})s", timescale)
+    num = int(match.group(1))
+    si_prefix = match.group(2)
+    exp = _si_prefix2exp[si_prefix]
+    return num * 10**exp, si_prefix
+
 
 class VCDDump(Dump):
-    def __init__(self, dump=None, timescale="1ps", comment=""):
+    def __init__(self, dump=None, samplerate=1e-12, timescale="1ps", comment=""):
         Dump.__init__(self)
         self.variables = [] if dump is None else dump.variables
         self.timescale = timescale
         self.comment = comment
         self.cnt = -1
+        # rescale the timescale from the provided one to one where it is equal to the samplerate
+        # this lets us output sequential change timestamps which helps with software like PulseView
+        # that slow down if a much smaller timescale than necessary is used
+        timescale_seconds, si_prefix = _timescale_str2num(timescale)
+        # factor of 2 scale is because of 2x samples from fake clock
+        self.count_timescale = int(1 / (timescale_seconds * samplerate * 2))
+        self.timescale_unit_str = si_prefix + "s"
 
     def change(self):
         r = ""
@@ -65,14 +89,12 @@ class VCDDump(Dump):
 
     def generate_timescale(self):
         r = "$timescale "
-        r += self.timescale
+        r += str(self.count_timescale) + self.timescale_unit_str
         r += " $end\n"
         return r
 
     def generate_vars(self):
-        r = "$scope "
-        r += self.timescale
-        r += " $end\n"
+        r = "$scope dumped_signals $end\n"
         for v in self.variables:
             r += "$var wire "
             r += str(v.width)