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Altera_Forum
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15 years ago

cmos vs ttl logic in schematics design

Hi, I am designing cyclone 3 FPGA board in which i have come across cmos logic circuit and ttl logic circuit. can i connect 3.3v of both the cmos and ttl logic? what are all the things we ...
Altera_Forum's avatar
Altera_Forum
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15 years ago

A 3.3V I/O standard can drive LVTTL, LVCMOS(both 3.3V supply) and TTL (5V supply). To drive 5V CMOS (e.g. 74HC family), you need a level converter. 74HCT logic has TTL levels on input and can be driven by the 3.3V I/O.

5V logic must not drive the FPGA without level conversion or voltage clamping.