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  1. #1
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    lvttl

    hi all,

    can anyone please point out to me the difference between the LVTTL and LVCMOS IO standard. Which one has higher risk of having its output data being distorted..?

    Thanks in advance.

    •   Alt30th October 2004, 05:33

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    lvttl levels

    LVTTL must have level complible with 5V TTL in Vol/Voh and Vil/Vih, which is only capable when the VCC is 3.3V. If the VCC is lower, TLL levels are no longer viable, so you only see LVCMOS such LVCMOS 2.5, 1.8, 1.5,... etc.


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    •   Alt31st October 2004, 04:51

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    lvttl lvcmos

    They differ by their input voltage requirement, and their output voltage specifications.

    Genuine TTL chips also took more current than CMOS, and could drive out more current. Today, most devices are CMOS, and many of those CMOS can give more current than the original CMOS devices. However, the voltage level that a CMOS and TTL need at input are different, and the voltage garanteed at output is also different, so the TTL and CMOS 'compatibility' now reffer mostly to those voltage levels.

    Just remember that even if a device have TTL-compatible IO, this doesn't meen that it is TTL. Original TTL were made of bipolar transistors, there were more immune to ESD (static discharge) and had a pull-up at input so you could let the input floating and the input would see a logic 1. CMOS chips of today, even if TTL-compatible IO, are made of MOSFET transistors, which need care when handling to not get zapped by ESD (do not handle them if you have a risk of zapping them with static), and you can NOT leave an input floating (not connected). Onless you internally enable a pull-up, which is usually applicable only in an FPGA, leaving in input floating will leave the input transistor in an unknown state (can randomly flip from a 0 to a 1 or vice-versa, take more current, even oscillation). Leaving an input floating can lead to some headhache in debugging. I remember those circuits, back in school, where I could see a LED turn on or off just by sweeping my hand near the chip, without even touching it!

    The LV versions reffer to the supply and IO levels lower than the original 5V. LVTTL is 3.3V. LVCMOS too, but can be even lower (2.5V, 1.8V, ...). An LVTTL can drive an original TTL because the output voltage specification of an LVTTL is within the TTL range. The LVCMOS also can drive a TTL and LVTTL. However, the output voltage specification of a LVTTL is not compatible with input specification of a CMOS, it may work for some design, and not for some other, or even work some day, to stop working on other days.

    CMOS have more strict requirement, where TTL is more relaxed. Originally, genuine TTL were more robust to signal degradations than CMOS, because of the more relaxed input requirement.

    To be strictly sure, you need to look at the AC characteristic in the devices datasheets, for the voltage and current specifications.


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