Instrument interrupt routines to output more debug information on PO…

…RTC.

    - Bit 0 is toggled when V-USB detects a SOF keep-alive pulse.
    - Bit 1 is set while V-USB is in its interrupt routine (i.e. processing and/or transmitting USB traffic).
    - Bit 2 is toggled on/off once at the end of every interrupt routine.

src/main.cpp

@@ -28,8 +28,8 @@ void setup()
     DDRB  |= 0b00111110;
     PORTB &= 0b11000001;
 
-    DDRC  |= 0b00000001;
-    PORTB &= 0b11111110;
+    DDRC  |= 0b00000111;
+    PORTC &= 0b11111000;
 
 
     Serial.begin(250000);

v-usb/usbdrv/asmcommon.inc

@@ -99,6 +99,9 @@ doReturn:
     rjmp    waitForJ            ;[51] save the pops and pushes -- a new interrupt is already pending
 sofError:
     POP_RETI                    ;macro call
+    cbi     PORTC, 1
+    sbi     PORTC, 2
+    cbi     PORTC, 2
     reti
 
 handleData:

v-usb/usbdrv/usbdrvasm16.inc

@@ -44,9 +44,12 @@ USB_INTR_VECTOR:
 ;YL is guarenteed to be < 0x80 because I flag is clear. When we jump to
 ;waitForJ, ensure that this prerequisite is met.
 waitForJ:
+    cbi     PORTC, 1
+    sbi     PORTC, 1
+waitForJForReal:
     inc     YL
     sbis    USBIN, USBMINUS
-    brne    waitForJ        ; just make sure we have ANY timeout
+    brne    waitForJForReal        ; just make sure we have ANY timeout
 waitForK:
 ;The following code results in a sampling window of < 1/4 bit which meets the spec.
     sbis    USBIN, USBMINUS     ;[-15]
@@ -239,7 +242,7 @@ bitstuffN:
     nop                     ;[9]
     out     USBOUT, x1      ;[10] <-- out
     rjmp    didStuffN       ;[0]
-    
+
 bitstuff6:
     eor     x1, x4          ;[5]
     ldi     x2, 0           ;[6] Carry is zero due to brcc