of Microprocessors

DIGITAL HARDWARE 591
Lsa
I
N+I DIVISOR INPUT
I
1 1
MSS
~--
CEP [A IS IC ID
..-- CEP IA IS IC [D
LCEP IA
IS f C fO
CET
CET
r-- CET
4:J LO 74161 TC- Lc LO 74161 TCU~ LD 74161 TC -
a CLR c CLR c CLR
CLK QA Qa Qc QD j CLK QA Qs Qc QD
rClK QA Qs Qc QD
MASTER
CLOCK
NOTE.
ALL UNUSED INPUTS TIED TO
A SOURCE OF LOGICAL ONE
DIVIDED
'------ FREQUENCY
OUTPUT
(NARROW PULSE)
Fig. 17-1. Simple, high-speed divide-by-N counter
The master clock may be as high as 15 MHz 00-40 MHz if "S" or "F"
series TTL logic is used), and the output frequency consists of pulses with a
length of one clock period. N may be changed at almost any time, but if it is
changed within 10 nsec of the end of an output pulse, an incorrect value may
be loaded into the counters and cause a momentary glitch (click) in the
output frequency.
The assumption behind the divide-by-N approach is that, if the clock
frequency is high enough, the frequency increment between adjacent values
of N will be small enough to be inaudible. Let's determine how high the
master clock must be to get a resolution of 5 cents (1/20 of a semitone or
0.3%) throughout the audio range. Taking the low end first, we seek a clock
frequency, F, such that FIN=20.0 and FI(N-1)=20.06, where N is the
division factor for a 20-Hz output. Using the standard procedure for solving
simultaneous equations, F and N are found to be 6.68 kHz and 334, respectively.
Solving the same equations at the upper end of the audio range gives
an F of 6.68 MHz and N the same as before.
Since the clock frequency must remain fixed, we are forced to use the
6.68 MHz value of F, since using lower values will not provide the required
amount of resolution at high frequencies. Thus, with a master clock of 6.68
MHz, the division ratio varies from 334 for a 20-kHz output to 334,000 for
a 20-Hz output. The counter circuit in Fig. 17-1 will therefore require 20
bits or five 4-bit counters, which could then provide frequencies as low as 6.4
Hz. While the resolution at high frequencies. barely meets specification, the
resolution at low frequencies is 1,000 times better than required. Note that
the high-frequency resolution can only be improved by a factor of two before
counter speed limitations become a factor. Clear!y, the divide-by-N method
of frequency generation performs best when generating low frequencies.
One advantage of the divide-by-N method is that the output frequency
is pure, that is, there is no jitter in the period other than that of the master