Specifications
CIRCUIT IDEAS
counter IC
CD4060, re-
petitively
outputting
the required
bit pattern,
correspond-
ing to bit pat-
tern for ‘Z’ in
this case.
Let us
see how the bitmap of a character is formed
to display any specific character. Here we
have used an 8 (rows) x 7 (columns) LED
display. We can use either a readymade
LED matrix display or assemble one our-
selves. Fig. 2 shows the LED pattern for
letter ‘A’ whose corresponding bitmap in
memory is shown in Table II. Each
memory location represents one column
of the display. Since seven columns are
used, we need seven locations (though
counter supplies eight locations/addresses)
for each character. The bitmap of each
character is stored in one memory page
(segment) of eight locations (8
th
location is
not used). The data from the correspond-
ing pages/locations are transferred to the
display by scanning the memory locations
and columns of the display simultaneously.
The EPROM used for the purpose is
27C32, with a memory capacity of 4 kB.
If you want to utilise its full capacity, you
can store codes for up to 512 characters.
The circuit shown here uses 1 kB of
memory space and can show up to 128
characters.
Each character bitmap is stored in a
memory page, and a particular memory
page (character) is selected by giving
its address. This page address is selected
via DIP switches S0 through S7. Each
page is scanned with the help of counter
CD4060, which has an inbuilt oscillator,
whose outputs are connected to A0, A1,
and A2 lines. The same lines are con-
nected to a decoder (4028) to drive the
columns of the display. The rows are con-
nected to the data outputs D0 through
D7 of the EPROM. Thus, when a memory
location is addressed, its data is output
on the corresponding column. The eight
memory locations corresponding to the se-
lected letter are consecutively scanned.
This process repeats itself at a fast rate.
Due to persistence of vision, one sees a
steady display of the corresponding
memory map.
Table III shows the data needed to be
stored in specific EPROM locations. It ca-
[0]
000 3C
001 42
002 81
003 81
004 82
005 42
006 3C
007 xx
[1]
008 00
009 00
00A 41
00B FF
00C 01
00D 00
00E 00
00F xx
[2]
010 61
011 83
012 85
013 89
014 91
015 61
016 00
017 xx
[3]
018 82
019 81
01A 81
01B 91
01C B1
01D D1
01E 8E
01F xx
[4]
020 08
021 18
022 28
023 48
024 BF
025 08
026 08
027 xx
[5]
028 E1
029 A1
02A A1
02B A1
02C A1
02D 92
02E 8C
02F xx
[6]
030 3C
031 4A
032 91
033 91
034 91
035 4A
036 24
037 xx
[7]
038 C1
039 82
03A 84
03B 98
03C 90
03D B0
03E C0
03F xx
[8]
040 6E
041 91
042 91
043 91
044 91
045 91
046 6E
047 xx
[9]
048 64
049 92
04A 91
04B 91
04C 91
04D 92
04E 7C
04F xx
[A]
050 1F
051 28
052 48
053 88
054 48
055 28
056 1F
057 xx
[B]
058 FF
059 91
05A 91
05B 91
05C 91
05D 91
05E 6E
05F xx
[C]
060 3C
061 42
062 81
063 81
064 81
065 42
066 24
067 xx
[D]
068 FF
069 81
06A 81
06B 81
06C 81
06D 42
06E 3C
06F xx
[E]
070 FF
071 91
072 91
073 91
074 91
075 81
076 81
077 xx
[F]
078 FF
079 90
07A 90
07B 90
07C 90
07D 80
07E 80
07F xx
[G]
080 3C
081 42
082 81
083 8D
084 89
085 4A
086 2C
087 xx
[H]
088 FF
089 10
08A 10
08B 10
08C 10
08D 10
08E FF
08F xx
[I]
090 00
091 00
092 81
093 FF
094 81
095 00
096 00
097 xx
[J]
098 86
099 81
09A 81
09B 81
09C FE
09D 80
09E 80
09F xx
[K]
0A0 FF
0A1 10
0A2 10
0A3 28
0A4 44
0A5 82
0A6 81
0A7 xx
[S]
0E0 62
0E1 91
0E2 91
0E3 91
0E4 91
0E5 91
0E6 4E
0E7 xx
[T]
0E8 80
0E9 80
0EA 80
0EB FF
0EC 80
0ED 80
0EE 80
0EF xx
[U]
0F0 FE
0F1 01
0F2 01
0F3 01
0F4 01
0F5 01
0F6 FE
0F7 xx
[V]
0F8 F8
0F9 04
0FA 02
0FB 01
0FC 02
0FD 04
0FE F8
0FF xx
[W]
100 FF
101 02
102 04
103 08
104 04
105 02
106 FF
107 xx
[X]
108 83
109 44
10A 28
10B 10
10C 28
10D 44
10E 83
10F xx
[Y]
110 80
111 40
112 20
113 1F
114 20
115 40
116 80
117 xx
[Z]
118 81
119 83
11A 85
11B 99
11C A1
11D C1
11E 81
11F xx
[+]
120 10
121 10
122 10
123
FE
124 10
125 10
126 10
127 xx
[–]
128 10
129 10
12A 10
12B 10
12C 10
12D 10
12E 10
12F xx
[÷]
130 10
131 10
132 10
133 54
134 10
135 10
136 10
137 xx
[x]
138 00
139 44
13A 28
13B 10
13C 28
13D 44
13E 00
13F xx
[=]
140 28
141 28
142 28
143 28
144 28
145 28
146 28
147 xx
Note: xx =
Don’t care
[L]
0A8 FF
0A9 01
0AA 01
0AB 01
0AC 01
0AD 01
0AE 01
0AF xx
[M]
0B0 FF
0B1 40
0B2 20
0B3 10
0B4 20
0B5 40
0B6 FF
0B7 xx
[N]
0B8 FF
0B9 40
0BA 20
0BB 18
0BC 04
0BD 02
0BE FF
0BF xx
[O]
0C0 3C
0C1 42
0C2 81
0C3 81
0C4 81
0C5 42
0C6 3C
0C7 xx
[P]
0C8 FF
0C9 90
0CA 90
0CB 90
0CC 90
0CD 90
0CE 60
0CF xx
[Q]
0D0 3C
0D1 42
0D2 81
0D3 81
0D4 85
0D5 42
0D6 3D
0D7 xx
[R]
0D8 FF
0D9 90
0DA 90
0DB 98
0DC 94
0DD 92
0DE 61
0DF xx
Address Data Address Data Address Data Address Data Address Data Address Data
TABLE III
Fig. 2: LED pattern for letter ‘A’
D7 0001 00 0X
D6 001 0 1 00X
D5 0 1 0001 0 X
D4 1 00000 1 X
D3 111111 1X
D2 1 00000 1 X
D1 1 00000 1 X
D0 1 00000 1 X
Address
Hexdata
Binary Data
TABLE II
050
051
052
053
054
055
056
057
1F
28
48
88
48
28
1F
XX
151