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ericmar
Joined: 14 Apr 2004
Posts: 279
Location: Singapore
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 01 Sep 2004 5:27 Problem about the 74LS90 Decade counter |
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Hi guys,
for some reasons the counter doesnt start from 0 but 8. It does count correctly except that it start from 8. I hv no idea why it is like this???
Thanks for any help.
Regards,
Eric
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echo47
Joined: 07 Apr 2002
Posts: 4205
Helped: 565
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| 01 Sep 2004 6:23 Problem about the 74LS90 Decade counter |
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| When you say "start", do you mean power-up? The 74LS90 does not have a power-up clear feature, so it powers-up into an undefined state. You must apply a reset pulse to guarantee zero.
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House_Cat
Joined: 21 Feb 2002
Posts: 1392
Helped: 292
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| 01 Sep 2004 6:39 Re: Problem about the 74LS90 Decade counter |
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There are two different resets on the 74LS90 - or I should say a master set and a master reset.
Pins 2 and 3 are master reset and will clear the 74LS90 to zero. High on pins 2 and 3 with pins 6 and 7 low will clear to zero.
Pins 6 and 7 are master set and will set the 74LS90 to nine. High on pins 6 and 7 with pins 2 and 3 low will set to nine.
Master set and master reset should be held low to count.
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