DEC VT220 Technical Manual page 131

The firmware in the 8051 8-bit microprocessor controls the
following three major keyboard operations at the same time.
1. Scans the keyboard to detect changes in the keyboard
matrix
2. Transmits results of the keyboard scan to the CPU
3. Receives information from the CPU
7.3.1.1 Keyboard Scanning -- The keyboard switches are connected
at the intersections of an 18
X
8 line matrix. This provides a
fixed position identifier for each key.
The firmware scans the lS-line axis and detects a pressed or newly
released key by reading the 8-line axis. The firmware then
verifies the detected keystroke and changes this position
information into an a-bit code that is unique to that key.
7.3.1.2 Control of Audio Transducer and Indicators -- Two circuits
control the audio transducer and the indicators. One circuit
receives its inputs from the 8051 and controls the transducer
(beeper). A long beep represents the bell and a short beep
represents the keyclick.
A separate circuit, controlled by a signal from the 8051, controls
each of the four indicators. The firmware, responding to commands
received from the CPU, turns the indicators on or off.
Keyboard Firmware Functions
This section describes the keyboard firmware functions. The
functions are divided into two categories: functions that cannot
be changed by CPU instructions and those that can.
7.3.2.1 Functions
following functions
CPU.
Power-Up test
Keycodes
Special codes
Not Changed by CPU Instructions The
cannot be changed by instructions from the
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