3.3.2.3 Explanations

The key to the previous code is the loop. Let take a look at the initialization, the loop itself, and its results.

db_or = 0x00;
db_and = 0xff;

This initializes the two variables we track in the loop. db_or is a cumulative bitwise-or ``sum'' (disjunction is somewhat analogous to addition in boolean algebra). The reason why each bit is initialized to 0 is simple: if any bit is initialized to 1, it will always be a 1, making the loop useless!

Likewise, db_and is a cumulative bitwise-and ``product''. All bits in this variable are initialized to 1 because if any were initialized to 0, it will always remain 0.

for (i = 0; i < db_n; ++i)
  {
    db_or |= db_buffer[i];
    db_and &= db_buffer[i];
  }

In the loop, both db_or and db_and are getting updated by the elements of db_buffer. Recall that db_buffer is a history of previous states of a port.

Let us think for a second here, not that I think you are starting to zone out. After the loop exits, how do we interpret each of the following?

• a bit value of 0 in db_or
• a bit value of 1 in db_or
• a bit value of 0 in db_and
• a bit value of 1 in db_and

Here is the answer.

• a bit value of 0 in db_or: the corresponding pin of the port has a complete history of 0.
• a bit value of 1 in db_or: the corresponding pin of the port has at least one sample of 1 in the history.
• a bit value of 0 in db_and: the corresponding pin of the port has at least one sample of 0 in the history.
• a bit value of 1 in db_and: the corresponding pin of the port has a complete history of 1.

Isn't this fantastic? Afterall, we were looking for consistent samples for the entire history buffer!

db_state |= (db_and);
  db_state &= (db_or);

Marks the end of this logic. db_state is the ``debounced state''. Each bit in this variable represents a filtered state of the corresponding pin of the port. If a pin has consistently read 1 for the whole history, it is safe to update db_state and make the corresponding bit a 1. This is done by a bitwise-or with db_and.

Similarly, if a pin has consistently read 0 for the whole history, it is safe to update db_state and make the corresponding bit a 0. This is done by a bitwise-and with db_or.

Q.E.D., quod erat demonstrandum (which was to be demonstrated, or in plain English, end of proof).