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Altera_Forum's avatar
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 have to taken care when go for different logics?

Thanx in advance :)

Regards,

Shahul

3 Replies

  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    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.