CONTROL REGISTER B:

  BIT  NAME     FUNCTION
  ---  ----     --------
   0   start    1 = start timer b, 0 = stop timer b.
                    This bit is automatically reset (= 0) when
                    underflow occurs during one-shot mode.
   1   pbon     1 = timer b output on pb7, 0 = pb7 is normal
                    operation.
   2   outmode  1 = toggle, 0 = pulse.
   3   runmode  1 = one-shot mode, 0 = continuous mode.
   4   load     1 = force load (this is a strobe input, there is no
                    data storage;  bit 4 will always read back a
                    zero and writing a 0 has no effect.)
  6,5  inmode   bits crb6 and crb5 select one of four possible
                input modes for Timer B, as follows:

                CRB6  CRB5   Mode Selected
                ----  ----   ---------------------------------------
                 0     0     Timer B counts 02 pulses
                 0     1     Timer B counts positive CNT transitions
                 1     0     Timer B counts Timer A underflow pulses
                 1     1     Timer B counts Timer A underflow pulses
                               while CNT pin is held high.

   7   alarm     1 = writing to tod registers sets alarm
                 0 = writing to TOD registers sets TOD clock.
                     Reading TOD registers always reads TOD clock,
                     regardless of the state of the Alarm bit.