diff --git a/tests/conftest.py b/tests/conftest.py
index f9127440..c444de09 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -8,7 +8,7 @@ def pytest_addoption(parser):
         default=False,
         help="Run tests with instruction tracing",
     )
- 
+
     parser.addoption(
         "--enablevcdtracing",
         action="store_true",
@@ -16,6 +16,7 @@ def pytest_addoption(parser):
         help="Run tests with vcd tracing",
     )
 
+
 @pytest.fixture
 def options(request):
     yield request.config.option
diff --git a/tests/test_sync_clk_basic.py b/tests/test_sync_clk_basic.py
index 1b01b049..8332f6c6 100644
--- a/tests/test_sync_clk_basic.py
+++ b/tests/test_sync_clk_basic.py
@@ -10,11 +10,11 @@ def test_file(request):
     return str(request.node.fspath)
 
 
-def do_test(test_file, options, capfd):
+def do_test(bus_speed, test_file, options, capfd):
 
     testname, _ = os.path.splitext(os.path.basename(test_file))
 
-    binary = f"{testname}/bin/{testname}.xe"
+    binary = f"{testname}/bin/{bus_speed}/{testname}_{bus_speed}.xe"
 
     tester = testers.ComparisonTester(open("pass.expect"))
 
@@ -25,7 +25,6 @@ def do_test(test_file, options, capfd):
         + "-port tile[0] XS1_PORT_1P 1 0 -port tile[0] XS1_PORT_1L 1 0 "
         + "-port tile[0] XS1_PORT_1A 1 0 -port tile[0] XS1_PORT_1B 1 0 "
     )
-    
 
     max_cycles = 15000000  # enough to reach the 10 skip + 100 test in sim at 48kHz
 
@@ -37,13 +36,21 @@ def do_test(test_file, options, capfd):
         loopback_args,
     ]
 
-    result = Pyxsim.run_on_simulator(binary, tester=tester, simargs=simargs, capfd=capfd, instTracing=options.enabletracing, vcdTracing=options.enablevcdtracing)
+    result = Pyxsim.run_on_simulator(
+        binary,
+        tester=tester,
+        simargs=simargs,
+        capfd=capfd,
+        instTracing=options.enabletracing,
+        vcdTracing=options.enablevcdtracing,
+    )
 
     return result
 
 
-def test_sync_clk_basic(test_file, options, capfd):
+@pytest.mark.parametrize("bus_speed", ["FS", "HS"])
+def test_sync_clk_basic(bus_speed, test_file, options, capfd):
 
-    result = do_test(test_file, options, capfd)
+    result = do_test(bus_speed, test_file, options, capfd)
 
     assert result
diff --git a/tests/test_sync_clk_basic/Makefile b/tests/test_sync_clk_basic/Makefile
index 274573d6..513f2614 100644
--- a/tests/test_sync_clk_basic/Makefile
+++ b/tests/test_sync_clk_basic/Makefile
@@ -1,7 +1,13 @@
 
 TEST_FLAGS ?=
 
-XCC_FLAGS = -O3 -g -DXUD_CORE_CLOCK=600 -save-temps -DUSB_TILE=tile[0] -DLOCAL_CLOCK_INCREMENT=10000 -DLOCAL_CLOCK_MARGIN=100 $(TEST_FLAGS)
+XCC_FLAGS_HS = -O3 -g -DXUD_CORE_CLOCK=600 -save-temps -DUSB_TILE=tile[0] -DLOCAL_CLOCK_INCREMENT=10000 -DLOCAL_CLOCK_MARGIN=100 \
+							-DBUS_SPEED=2 \
+							$(TEST_FLAGS)
+
+XCC_FLAGS_FS = -O3 -g -DXUD_CORE_CLOCK=600 -save-temps -DUSB_TILE=tile[0] -DLOCAL_CLOCK_INCREMENT=10000 -DLOCAL_CLOCK_MARGIN=100 \
+							-DBUS_SPEED=1 \
+							$(TEST_FLAGS)
 
 TARGET = test_xs3_600.xn
 
diff --git a/tests/test_sync_clk_basic/src/main.xc b/tests/test_sync_clk_basic/src/main.xc
index 65864421..6a7531e8 100644
--- a/tests/test_sync_clk_basic/src/main.xc
+++ b/tests/test_sync_clk_basic/src/main.xc
@@ -1,30 +1,44 @@
 // Copyright 2022 XMOS LIMITED.
 // This Software is subject to the terms of the XMOS Public Licence: Version 1.
 
-/* Simples test to ensure reference clock to CS2100 device continues when SOF clock not available
- * Note, this test uses "nice" numbers and it doesn't check for graceful change over from internal to SOF clock 
+/* Simple test to ensure reference clock to CS2100 device continues when SOF clock not available
+ * Note, this test uses "nice" numbers (i.e. MISSIG_SOFS %8 == 0) and therefore doesn't check 
+ * for a graceful change over from internal to SOF clock 
  */
 
 #include "platform.h"
 #include "xua.h"
-#include "uac_hwresources.h"
 #include "print.h"
+#include "xud.h"
 
-#define EP_COUNT_IN 3
-#define EP_COUNT_OUT 3
+#define EP_COUNT_IN        (3)
+#define EP_COUNT_OUT       (3)
 
-out port p_pll_ref               = XS1_PORT_1A; 
-in port p_off_mclk               = XS1_PORT_1M;
-in port p_pll_loop               = XS1_PORT_1B; /* Note, this is externally looped back using the loopback plugin */
+out port p_pll_ref         = XS1_PORT_1A; 
+in port p_off_mclk         = XS1_PORT_1M;
+in port p_pll_loop         = XS1_PORT_1B; /* Note, this is externally looped back using the loopback plugin */
 
 /* Purely for debug/viewing on VCD */
-out port p_test                  = XS1_PORT_1C;
+out port p_test0           = XS1_PORT_1C;
+out port p_test1           = XS1_PORT_1D;
+
+#ifndef BUS_SPEED
+#error BUS_SPEED should be defined
+#endif
 
 /* To speed this test up we divide all delays by 10. This is also the case for the delays in the clock generation code */
-#define SOF_PERIOD_TICKS   (12500/10)
+#if(BUS_SPEED == 2) // XUD_SPEED_HS
+    #define SOF_PERIOD_TICKS   (12500/10)
+    #define SOF_DIVIDE         (1)
+#else
+    #define SOF_PERIOD_TICKS   ((12500*8)/10)
+    #define SOF_DIVIDE         (8)
+#endif
 
-#ifndef MISSING_SOFS
-#define MISSING SOFS (8)
+#ifndef MISSING_SOF_PERIOD
+/* By default skip a whole number of SOF periods (easy case)
+ * Note, app_test_sync_plugin/Makefiles sets this to something more nasty */
+#define MISSING_SOF_PERIOD       (8 * SOF_PERIOD_TICKS)
 #endif
 
 void exit(int);
@@ -45,18 +59,19 @@ void AudioHwInit()
     return;
 }
 
-void AudioHwConfig(unsigned samFreq, unsigned mClk, unsigned dsdMode,
-    unsigned sampRes_DAC, unsigned sampRes_ADC)
+void AudioHwConfig(unsigned samFreq, unsigned mClk, unsigned dsdMode, unsigned sampRes_DAC, unsigned sampRes_ADC)
 {
     return;
 }
 
 void driveSofs(chanend c_sof, int count)
 {
-    static int sof = 0;
+    static int frame = 0;
+
+    /* For HS frame should only increment every 8 SOFs, but this is a "dont care" */
     for(int i = 0; i < count; i++)
     {
-        outuint(c_sof, sof++);
+        outuint(c_sof, frame++);
         delay(SOF_PERIOD_TICKS);
     }
 }
@@ -66,11 +81,12 @@ void fake_xud(chanend c_out[], chanend c_in[], chanend c_sof)
     timer t;
     unsigned time;
   
-    p_test <: 1;
 
     /* Makes traces a bit nicer to look at */ 
     t :> time;
     t when timerafter(SOF_PERIOD_TICKS * 2) :> int x;
+    
+    p_test0 <: 1;
    
     /* Endpoint type tables */
     XUD_EpType epTypeTableOut[EP_COUNT_OUT] = {XUD_EPTYPE_CTL, XUD_EPTYPE_ISO, XUD_EPTYPE_DIS};
@@ -78,38 +94,48 @@ void fake_xud(chanend c_out[], chanend c_in[], chanend c_sof)
  
     SetupEndpoints(c_out, EP_COUNT_OUT, c_in, EP_COUNT_IN, epTypeTableOut, epTypeTableIn);
 
-    driveSofs(c_sof, 32);
+    driveSofs(c_sof, 32/SOF_DIVIDE);
      
-    p_test <: 0;
+    p_test0 <: 0;
     
     /* Sim missing SOFs */   
-    delay(SOF_PERIOD_TICKS*MISSING_SOFS);
+    delay(MISSING_SOF_PERIOD);
     
-    p_test <: 1;
+    p_test0 <: 1;
     
-    driveSofs(c_sof, 16);
+    driveSofs(c_sof, 16/SOF_DIVIDE);
+    
+    p_test0 <: 0;
+    
+    delay(MISSING_SOF_PERIOD);
+    
+    p_test0 <: 1;
+    
+    driveSofs(c_sof, 16/SOF_DIVIDE);
+    
+    p_test0 <: 0;
     
-    p_test <: 0;
 }
 
 extern XUD_BusSpeed_t g_curUsbSpeed;
 
 #define MARGIN (1500/10)
-#define EXPECTED_PERIOD (100000/10) // Test runs as HS - so 8 * 125000
+#define EXPECTED_PERIOD (100000/10)
 
 void checker()
 {
     timer t; 
     unsigned t0, t1;
     unsigned x = 0;
-
     int fail = 0;
 
+    p_test1 <: 1;
+
     p_pll_loop when pinseq(1) :> x;
     p_pll_loop when pinseq(0) :> x;
     p_pll_loop when pinseq(1) :> x;
 
-    for(int i = 0; i < 8; i++)
+    for(int i = 0; i < 12; i++)
     {
         p_pll_loop when pinsneq(x) :>  x;
         t :> t0;
@@ -135,6 +161,8 @@ void checker()
     if(!fail)
         printstrln("PASS");
 
+    p_test1 <: 0;
+
     exit(0);
 }
 
@@ -153,7 +181,8 @@ int main()
         PllRefPinTask(i_pll_ref, p_pll_ref);
 
         {
-            g_curUsbSpeed = XUD_SPEED_HS;
+            g_curUsbSpeed = BUS_SPEED;
+
             XUA_Buffer_Ep(c_out[1],      /* USB Audio Out*/
                 c_in[1],                 /* USB Audio In */
                 c_sof, c_aud_ctl, p_off_mclk, i_pll_ref
diff --git a/tests/test_sync_clk_plugin.py b/tests/test_sync_clk_plugin.py
index 628484e8..8da6c8c2 100644
--- a/tests/test_sync_clk_plugin.py
+++ b/tests/test_sync_clk_plugin.py
@@ -1,19 +1,10 @@
 import pytest
-import Pyxsim
-from Pyxsim import testers
-import os
-import sys
-
 from test_sync_clk_basic import do_test
+from test_sync_clk_basic import test_file
 
-@pytest.fixture()
-def test_file(request):
-    return str(request.node.fspath)
-
-def test_sync_clk_plugin(test_file, options, capfd):
-
-    #pytest.xfail("This is a known failure due to issue #275")
-
-    result = do_test(test_file, options, capfd)
 
+@pytest.mark.parametrize("bus_speed", ["FS", "HS"])
+def test_sync_clk_plugin(bus_speed, test_file, options, capfd):
+    pytest.xfail("This is a known failure due to issue #275")
+    result = do_test(bus_speed, test_file, options, capfd)
     assert result
diff --git a/tests/test_sync_clk_plugin/Makefile b/tests/test_sync_clk_plugin/Makefile
index e02143c8..08eec76a 100644
--- a/tests/test_sync_clk_plugin/Makefile
+++ b/tests/test_sync_clk_plugin/Makefile
@@ -1,3 +1,4 @@
-TEST_FLAGS = -DMISSING_SOFS=4
+
+TEST_FLAGS = -DMISSING_SOF_PERIOD=7500
 
 include ../test_sync_clk_basic/Makefile
diff --git a/tests/test_sync_clk_plugin/src/main.xc b/tests/test_sync_clk_plugin/src/main.xc
index b968732b..f6e3d50a 100644
--- a/tests/test_sync_clk_plugin/src/main.xc
+++ b/tests/test_sync_clk_plugin/src/main.xc
@@ -1,2 +1,6 @@
 
+/* This tests for a smooth transition from internal to SOF clock.
+ * Unlike test_sync_clk_basic it does not use a "nice" value for MISSING_SOFS
+ */
+
 #include "../test_sync_clk_basic/src/main.xc"