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Fig. 1. Schematic. Fo r U2, U3, U9, U10, U11, and U12, Vcc = pin 14 and ground = pin 7. No te: Two gates in U3 and two
in vert ers in U9 are not used; gro und all unu sed inputs.
l 57031 is described be low.
It has l -Hz resol ution on
d irect o r la-Hz resol ution
on prescale using 'l -second
gate time, or l a-Hz reso lution
o n d irect an d 100-Hz
resoluti on on prescale us­
109 .1-seco nd gati ng. The
di rect inp ut is go od to at
least 50 MHz , and th e prescaled
input should exceed
500 MHz. The counte r has
leading 0 bla nking, and, if
turned o n w ith no in pu t
signa l, will just d isplay a 0
at the least significant digit
po sition a long with the decim
al po int to remind you
what range yo u are on. In
the direct mode, the decimal
po int is placed to read
in kHz; in the prescale
mode, the d ispl ay is in
MHz.
Circuitry
As can be seen in the
block diagram, the l57031
gre atl y simpl ifies th e circuit
ry. The pr esc aler is an
llC90 EC l IC wh ich
di vides the input signa l by
10 and o ut puts in TTL. The
d irect in pu t pr eamp was
taken from " The l atest in
l S 7031
Cou nters" by WA1UFE, in
t he December, 1976, issue
of 73.
The input-se lect switch
chooses t he source. Th is
signal is gated by U3 dur ing
the 0 to 9 counts of U'l . At
count 10 , t he l o ad input to
I CO D.lTA
BCD TO
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DECODER I DRIVER
Fig. 2. Block dia gram of LSI-based co unter.
U6 goes low, latch ing in
eac h d igit's value and displaying
it, and at co unt 11,
the Reset line goes low,
resetting to 0 a ll the counters,
both intern a l and exte r­
na l. Then it beg ins to tabulat
e a new value du rin g t he
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73 Magazine. October,1980 137