On ATtiny84, why does delay() stop working properly when RadioHead ASK is used?

Question

When I call the delay() function on the ATtiny84 micro, it delays for the expected time, until I call the init() function on an RH_ASK object. After this, the micro seems to freeze.

#include <RH_ASK.h>
RH_ASK driver();
void setup() {
 delay(100); // works ok
 driver.init();
 delay(100); // freezes a long time (or maybe forever?)
}

Answer 1

After reading the source code, RH_ASK.cpp, I found a commented out macro.

// RH_ASK on ATtiny8x uses Timer 0 to generate interrupts 8 times per 
// bit interval. Timer 0 is used by Arduino platform for millis()/micros() 
// which is used by delay()
// Uncomment the define RH_ASK_ATTINY_USE_TIMER1 bellow, if you want to use 
// Timer 1 instead of Timer 0 on ATtiny
// Timer 1 is also used by some other libraries, e.g. Servo. Alway check 
// usage of Timer 1 before enabling this.
// Should be moved to header file
//#define RH_ASK_ATTINY_USE_TIMER1

In other words, the default behaviour of RadioHead ASK init() is to use Timer 0, which causes delay() (which uses millis()/micro()) to behave incorrectly.

To fix: Tell RadioHead ASK to use Timer 1. Because the RH_ASK_ATTINY_USE_TIMER1 macro is in the source and not the header (for now), you have to modify the actual RadioHead ASK source file, RH_ASK.cpp*. Un-comment the line #define RH_ASK_ATTINY_USE_TIMER1 which will use Timer 1 instead of Timer 0. Re-compile your code, and delay() should start working properly again.

* RH_ASK.cpp can be found in %USERPROFILE%\Documents\Arduino\libraries\RadioHead on Windows, if you installed RadioHead via the Arduino IDE library manager.