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  #1  
Old 12-21-2007, 05:15 AM
k.vinay.nayak k.vinay.nayak is offline
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Default Swiching level voltage values for TTL and CMOS.

Is VIH: [Voltage Input High],VIL: [Voltage Input Low], VOL: [Voltage Output Low], VOH: [Voltage Output High] ,VT: [Threshold Voltage] values are same for 5V TTL and 3.3V TTL or not?

Also these [Voltage Input High],VIL: [Voltage Input Low], VOL: [Voltage Output Low], VOH: [Voltage Output High] ,VT: [Threshold Voltage] values are different for CMOS Gates compared to TTL Gates?

Please help me to get clarified.

Thanks
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Old 12-21-2007, 08:54 AM
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I don't know about the 3.3V TTL, but logic levels are different when comparing TTL with CMOS. Normally a TTL gate needs a lower voltage than a CMOS gate to assume logic high in that input. The output voltage of a TTL gate driven high might be lower than the output voltage of a CMOS gate in the same situation. It even might be lower than the required voltage to drive a CMOS input high. That is why a TTL gate wont reliably drive a CMOS gate without a pull-up resistor.

Here is detailed information about the logic levels of the different families:
http://www.interfacebus.com/voltage_threshold.html

Notice that the 74HCT family is not a TTL family. It is a CMOS family with TTL compatible inputs. It is the only CMOS family that can be reliably driven by TTL chips.
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Last edited by cumesoftware; 12-21-2007 at 08:56 AM.
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Old 12-21-2007, 09:52 PM
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There is no such thing as 3.3 Volt TTL. TTL requires that Vcc be in the range 4.5 V to 5.5V

3.3Volt operation is a CMOS thing
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Old 12-22-2007, 05:23 PM
Distort10n Distort10n is offline
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Quote:
Originally Posted by Papabravo View Post
There is no such thing as 3.3 Volt TTL. TTL requires that Vcc be in the range 4.5 V to 5.5V

3.3Volt operation is a CMOS thing
Then what is LVTTL?
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Old 12-22-2007, 11:02 PM
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You tell me. I've never used it. Is there some reason I should?
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Old 12-23-2007, 12:14 AM
Distort10n Distort10n is offline
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You don't have to use it. Just so you know that there is such a thing as 3.3V TTL.
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Old 12-23-2007, 03:11 AM
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Does it have any advantages? Is there a full complement of parts in the family?
Apparently not. It consists of interface buffers, some octal flip flops and latches, no gates or decoders or multiplexers at least none in the TI lineup. Hardly worth a second thought.
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Last edited by Papabravo; 12-23-2007 at 03:19 AM.
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Old 12-23-2007, 03:06 PM
Distort10n Distort10n is offline
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...and clock buffers/generators, transcievers, some gates. Many of the newer logic families do not have a full compliment of devices; e.g., AUC logic.
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Old 12-24-2007, 10:26 AM
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So why would you use these parts given the other choices? Just because they are there seems somehow less than sufficient.
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Old 12-24-2007, 02:23 PM
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You guys apparently know of companies that actually make LVTTL products. Who are they? My searches found lots of devices with LVTTL level compatibility, and LVTTL to (insert logic family) translators, etc., but I didn't find any that were LVTTL in and LVTTL out, made with a bipolar process.
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