About the Timer/Counter Inclass assignment "heartbeat":

"heartbeat" does not require any hardware preparation. However, it 
is good to carefully consider what it is you are trying to do with 
this code. It's not immediately obvious.

To make the heartbeat pattern on a PORTB LED, we must vary the brightness
of the LED from dim to bright and back to dim. This action is repeated
forever. A PWM varies the average current through the LED by changing 
the duty cycle of its output. This method for dimming the LED is far
more efficient than linearly varying the current through a transistor.

We will use TCNT1 as the PWM generator. TCNT1's OC1A output is the
PWM output signal available at PB5. 
OC1A = Output Compare Register 1, output A. 
(There are three output compare registers; A, B and C)

So that we cannot see any "flickering" from the PWM output, set the
PWM frequency to about 260Hz. This is well above a human eye's ability 
to see. We cannot even see 60Hz flickering in most cases. 100Hz is 
pretty much "flicker-free", and 260Hz is a safe bet.

Now, how much do we change the PWM duty cycle at each update? We want 
a smooth, seemingly continuous change in brightness. We have to balance 
two factors; the update rate of the PWM and the jump size at each update. 
These two factors need to yield a heartbeat of 60 beats/min.

One last issue is the logarithmic response to brightness of the human eye. 
If our steps are linear, the heartbeat won't look quite right. The solution 
for this is to "eyeball" (!) how it looks and change values accordingly. 

One "bright spot" in all this (another pun!) is that the LED's brightness 
is linear with respect to the current through it.

Who would have guessed how much subtlety there was in a simple
heartbeat function?