Hardware Reference
In-Depth Information
Figure 15.7. Setting the keyboard repeat delay and repeat rate in Windows.
Note
The increments on the Repeat Delay and Repeat Rate sliders in Keyboard Properties in the
Control Panel correspond to the timings given for the MODE command's RATE and DELAY
values. Each mark in the Repeat Delay slider adds about 0.25 seconds to the delay, and the
marks in the Repeat Rate slider are worth about one character per second (1cps) each.
The dialog box also contains a text box you can use to test the settings you have chosen before
committing them to your system. When you click in the box and press a key, the keyboard reacts using
the settings currently specified by the sliders, even if you have not yet applied the changes to the
Windows environment.
Keyboard Scan Codes
When you press a key on the keyboard, the processor built in to the keyboard (8048 or 6805 type)
reads the keyswitch location in the keyboard matrix. The processor then sends to the motherboard a
serial packet of data containing the scan code for the key that was pressed.
This is called the
Make code
. When the key is released, a corresponding
Break code
is sent,
indicating to the motherboard that the key has been released. The Break code is equivalent to the
Make scan code plus 80h. For example, if the Make scan code for the A key is 1Eh, the Break code
would be 9Eh. By using both Make and Break scan codes, the system can determine whether a
particular key has been held down and determine whether multiple keys are being pressed.
Many keyboards feature hotkeys that either have fixed uses—such as opening the default web
browser, sending the system into standby mode, and adjusting the speaker volume—or are
