Making Things Talk

68 MAKING THINGS TALK

Flow Control

You may notice that the paddles don’t move as smoothly onscreen as Monski’s arms

move. Sometimes the paddles seem not to move at all for a fraction of a second,

and sometimes they seem to lag behind the actions you’re taking. This is because

the communication between the two devices is asynchronous.

Although they agree on the rate at which data is

exchanged, it doesn’t mean that the receiving computer’s

program has to use the bits at the same time as they’re

sent. Monitoring the incoming bits is actually handled by

a dedicated hardware circuit, and the incoming bits are

stored in a memory buffer called the serial buffer until

the current program is ready to use them. Most personal

computers have a buffer that can hold a couple thousand

bytes. The program using the bits (Processing, in the

previous example) is handling a number of other tasks, like

redrawing the screen, handling the math that goes with it,

and sharing processor time with other programs through

the operating system. It may get bytes from the buffer less

than a hundred times a second, even though the bytes are

coming in much faster.

There’s another way to handle the communication

between the two devices that can alleviate this problem.

If Processing asks for data only when it needs it, and if

the microcontroller only sends one packet of data when

it gets a request for data, the two ...