Timer
-----

  Sometimes it's useful to have a timer that interrupts at
 regular intervals for routines that require periodic or
 percise updates. The timer in the GameBoy has a selectable
 frequency of 4096, 16384, 65536, or 262144 Hertz. This
 frequency increments the Timer Counter (TIMA). When it
 overflows, it generates an interrupt. It is then loaded
 with the contents of Timer Modulo (TMA). The following
 are examples:

 ;This interval timer interrupts 4096 times per second

     ld  a,-1
     ld  ($FF06),a     ;Set TMA to divide clock by 1
     ld  a,4
     ld  ($FF07),a     ;Set clock to 4096 Hertz

 ;This interval timer interrupts 65536 times per second

     ld  a,-4
     ld  ($FF06),a     ;Set TMA to divide clock by 4
     ld  a,5
     ld  ($FF07),a     ;Set clock to 262144 Hertz