VisualMicro Can't compile in debug ('Serial' was not declared in this scope) for attiny84a
'Serial' was not declared in this scope


This error is not surprising when compiling for ATtiny84 because in Arduino Serial refers to the hardware serial port and ATtiny84 doesn't have one.

But you didn't use Serial in your sketch so why should you get this error? The answer is this magical debug feature of Visual Micro that you desire. It turns out that on AVR microcontrollers such as your ATtiny84 all this actually consists of is Visual Micro automatically inserting various Serial.print() statements into your code, just as we always do manually to debug our code in the good old Arduino IDE:

http://www.visualmicro.com/page/User-Guide.aspx?doc=Debugging-Explained.html

So in fact your sketch is attempting to use Serial without your knowledge or being able to see it! If you turn off debugging you should no longer get that error.

Visual Micro does allow you to change the debugging output to software serial:

http://www.visualmicro.com/page/User-Guide.aspx?doc=Debugging-With-Different-Ports.html

but since you say you have no pins available that may not be any help.