Set mux = 1 during vsync detection to keep clamp pulses active

src/macros.S

@@ -63,6 +63,13 @@ waithiF\@:
 .endm
 
 .macro  SWITCH_PSYNC_TO_VSYNC
+        ldr    r8, =GPLEV0
+        ldr    r9, [r8]
+        push   {r9}
+
+        ldr    r8, =GPSET0
+        mov    r9, #1 << MUX_PIN
+        str    r9, [r8]                          // set MUX = 1
         ldr    r8, =GPCLR0
         mov    r9, #1 << VERSION_PIN
         str    r9, [r8]                          // set version = 0
@@ -71,7 +78,13 @@ waithiF\@:
 .macro  SWITCH_VSYNC_TO_PSYNC
         ldr    r8, =GPSET0
         mov    r9, #1 << VERSION_PIN
-        str    r9, [r8]                          // set version = 1
+        str    r9, [r8]                         // set version = 1
+        ldr    r8, =GPCLR0
+        
+        pop    {r9}
+        tst    r9, #MUX_PIN
+        moveq  r9, #1 << MUX_PIN
+        streq  r9, [r8]                         // restore MUX if zero
 .endm
 
 // Wait for the next edge on psync