Ch06b-DesignTopics

Ch06b-DesignTopics - Clark Guest 2009 ECE 25 Logic and...

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©Clark Guest 2009 ECE 25 Logic and Computer Design Synchronous vs Asynchronous 9 Synchronous Asynchronous Synchronous Synchronous Synchronous Asynchronous Synchronous Synchronous Clock Clock Clock Synchronous: All fip-fops have the same ungated clock Asynchronous: Flip-fops have di±±erent or gated clocks ©Clark Guest 2009 ECE 25 Logic and Computer Design Why Asynchronous is Bad 10 D C CLR D C CLR Clock In (asynchronous) ? ? t s t h Clock In ©Clark Guest 2009 ECE 25 Logic and Computer Design Fixing Asynchronous (Sort o±) 11 D C CLR D C CLR Clock In (asynchronous) D C CLR Both 0 or both 1 ©Clark Guest 2009 ECE 25 Logic and Computer Design Metastability 12 Like fipping a coin, a fip-fop should come down in one o± two states. But sometimes it lands on its edge, and balances ±or a while. Likely to happen i± setup or hold times are violated. t pd t s t pd t s Clock D Q Normal Metastable
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©Clark Guest 2009 ECE 25 Logic and Computer Design Suicide Counter (Bad!) 13 D0 D1 D2 D3 CLK Q0
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Ch06b-DesignTopics - Clark Guest 2009 ECE 25 Logic and...

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