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4-4
Custom IC (U4)
Table 4-2 Pin Assignments
for Custom IC U4
Theory of Operations
This custom IC is surface-mounted in a 100-pin package. Vee
and ground each use four pins; the remaining pins are signal
lines.
Major functions of the custom IC are as follows:
• Generates frame timing, both horizontal and vertical
• Contains a three-tone beeper driver
• Provides nonhidden and hidden attribute control circuits
• Provides video shift register and half-clock dot stretch
• Generates a font-selection signal
• Generates control signals for the display RAM
• Provides smooth scroll control
• Generates character and cursor blink rate
• Arbitrates memory control between the display and the
microcontroller without disturbing the display
• Latches the address from the microcontroller for the EPROM
Pin assignments for custom IC U4 are given in Table 4-2.
Cescrlptlon
Pin
Clrectlon
+S V
16
+S V
44
+S V
68
+S V
88
Ground
17
Ground
46
Ground
66
Ground
90
RD
SS
In
WR
S1
In
ALE
28
In
Beeper
6S
Out
Keyboard command
Sl
Out
Attribute write
1
Out
Character write
98
Out
Font write
3
Out
Parallel port enable (PPE)
s
Out
Dot clock
63
In
H-Sync
S3
Out
V-Sync
59
Out
Font address 0-12
4, 6-15, 18, 20
Out
Font data 0-7
19, 21-27
In/Out
Character data 0-7
73, 7S, 76, 78,
In/Out
80-83
Attribute data 0-7
67, 69-72, 74,
In/Out
77, 79
Address 0-7
34, 36, 38,
Out
40-43, 4S
Address 8-1 S
29-33, 3S, 37, 39
In
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