Page 113 - Mechanical Engineers Reference Book

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T

AA B(

J
(C J
m Q+b - (c Q --o 4- J Q
Input ~ CK CK CK
clock -K -K -K

Figure 2.114 Asynchronous, 3-bit binary counter

CK
CK
D
QA
Figure 2.116 D-type flip-flop

manufactured as an 8-pin DIL package. The CMOS equiva-
lent to the TTL-based NE555 is the ICM7555. These packages
are essentially identical, and ICM7555 can be used to replace
NE555, although the converse is not always applicable. The
so-called ‘555’ timer is very versatile and can be used in either
’monostable’ or ’astable’ mode.
2.3.32.1 Monostable
Figure 2.117 shows the 555 wired up for monostable opera-
tion. When the ‘trigger’ is taken from +5 V to 0 V (i.e. high to
low) the output will go high for a period determined by the
values selected for R and C. The length of the output pulse is
given by 1.1RC.
F I ip-f op
I
Clock
pulses 7’””
C B A
0 0 0 0
1 0 0 1
2 0 1 0
3 0 1 1
4 1 0 0
5 1 0 1
6 1 1 0
7 1 1 1
0 0 0 0

Figure 2.115 State table and timing diagram for a 3-bit binary Trigger
counter *

2.3.32 Timers and pulse circuits -- I
An essential feature of the flip-flop circuits described in the
previous two sections was the provision of a pulsed clock
signal. Although timers can be designed using discrete compo-
nents, it is normal to design round the commonly available
timers which are already available in IC form. The most
prevalent timer currently in use is the NE555. which is Figure 2.117 555 timer in monstable operation