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TTL sinking and sourcing

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nvd

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What is the current required by a single TTL input while sourced or sunk.

Can AT89S52 be used with an LED directly without damaging the port pin while the port is sourcing.

Regards...
 

IanP

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Typical TTL input voltages and currents are as follow:

Vih 2V (min)
Vil 0.8V (max)
Iih 20uA (min)
Iil -0.2mA (max)

One 89S52 output can sink ≈10mA so it will easily drive LED(s) .. but the same pin can only source currents of hundreds of µAs and that is not enough to drive LEDs ..
Connecting LED between any pin and GND will not damage that pin ..

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IanP
 

    nvd

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Naveed Alam

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IanP said:
Typical TTL input voltages and currents are as follow
its never feasible to drive any component directly from ICs (especially controllers..) its never recommended..

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nvd

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But I think Microcontroller won't be damaged if it is used within its limits as specified by its datasheet.
 

Sceadwian

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Naveed, that is a completly erroneous statement, it is very safe to drive MANY different types of components directly from an MCU I/O line, as long as max/min values are adhered to, and you're not driving an inductive load.
 

Gorgon

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nvd said:
What is the current required by a single TTL input while sourced or sunk.

Can AT89S52 be used with an LED directly without damaging the port pin while the port is sourcing.

Regards...
Hi,
TTL can be many types of interfaces. If you use the old standard TTL chips with no extra letters like 7400, you'll need 400uA source and -1.6mA sink current for each input.
The lowpower version of TTL, the LS, like 74LS00 need 40uA source and -800uA(max) sink.
The cmos version 74HC(T)00 has almost no input current at all, around +/-1uA.

There are other variants also, but these are the most common ones.

When driving anything from an output you need to study the datasheet for that specified output. Many micros has special driver outputs on some pins.

One thing to remember if you plan to drive leds and logic inputs from the same pin, is the testcondition for the specified load.
When a datasheet tell you that you can load an output with 25mA max, it may be that the output is then offset by half the supply voltage, and of no use for driving logic a the same time.
Another parameter is the capacitance on an input, if you use many inputs on one output, you may have problems with the dynamic load on the output. In short, it takes time to switch the load, and you get rotten flanks on your high speed signal.

The datasheet is made for you to read, even if you only tinker with the micro as a hobby.

TOK ;)
 

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