of Microprocessors

634 MUSICAL ApPLICATIONS OF MICROPROCESSORS
are also clocked by the prescaler output. This prevents its content from
changing at inopportune times and glitching the adders or waveform
generator. Since the 74173 is a synchronous load register, it must be enabled
by the control computer long enough to be loaded by the next clock pulse.
Even with the largest prescaler division rario selected, the load delay is never
more than 15 J.Lsec.
Symmetry logic and waveform partitioning logic has been incorporated
into a single 256-word X 4-bit programmable read-only memory. Three of
the ROM address bits are the three most significant bits of the postdivider,
whereas the remaining five address bits are the waveform mode control bits.
Thus, one of 32 different waveform memory operating modes can be
selected. This allows near!y all combinations of waveform table size (64, 128,
or 256), waveform selection when the smaller sizes are used, and symmetry
to be specified. Two of the ROM output bits control conditional com-
Table 17-2. Waveform Memory Scan Control PROM Pattern
Mode Most significant accumulator bits 2 Mode
Number' 0 1 2 3 4 5 6 7 description
0 0 3 0 0 0 0 0 0 0 64 points, wave 1, no symmetry
1 0 4 0 4 0 4 0 4 even symmetry
2 0 8 0 8 0 8 0 8 odd symmetry
3 0 C 8 4 0 C 8 4 odd-even symmetry
4 0 4 8 C 0 4 8 C even-odd symmetry
5 1 1 1 1 1 1 1 1 wave 2, no symmetry
6 1 5 1 5 1 5 1 5 even symmetry
7 1 9 1 9 1 9 1 9 odd symmetry
8 1 D 9 5 1 D 9 5 odd-even symmetry
9 1 5 9 D 1 5 9 D even-odd symmetry
10 2 2 2 2 2 2 2 2 wave 3, no symmetry
11 2 6 2 6 2 6 2 6 even symmetry
12 2 A 2 A 2 A 2 A odd symmetry
13 2 E A 6 2 E A 6 odd-even symmetry
14 2 6 A E 2 6 A E even-odd symmetry
15 3 3 3 3 3 3 3 3 wave 4, no symmetry
16 3 7 3 7 3 7 3 7 even symmetry
17 3 B 3 B 3 B 3 B odd symmetry
18 3 F B 7 3 F B 7 odd-even symmetry
19 3 7 B F 3 7 B F even-odd symmetry
20 0 1 0 1 0 1 0 1 128 points, wave 1, no symmetry
21 0 1 4 5 0 1 4 5 even symmetry
22 0 1 8 9 0 1 8 9 odd symmetry
23 0 1 C D 8 9 4 5 odd-even symmetry
24 0 1 4 5 8 9 C D even-odd symmetry
25 2 3 2 3 2 3 2 3 wave 2, no symmetry
26 2 3 6 7 2 3 6 7 even symmetry
27 2 3 A B 2 3 A B odd symmetry
28 2 3 E F A B 6 7 odd-even symmetry
29 2 5 6 7 A B E F even-odd symmetry
30 0 1 2 3 0 1 2 3 256 points, wave 1, no symmetry
31 0 1 2 3 4 5 6 7 even symmetry
Notes: 'Most significant 5 PROM address bits expressed in decimal.
2Least significant 3 PROM address bits expressed in decimal.
'PROM contents expressed in hexadecimal.