Electrical scheme of the output pin

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Jan Lichtenbelt
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Electrical scheme of the output pin

Post by Jan Lichtenbelt »

Hi,

1) Where can I find the electrical schem of the output pin, TTL or Schmitt Trigger? Espeacillay for the PIC16F876 or equivalent.

2) Is there a typical load resitance of 230 Ohm?


with kind reagrds

Jan Lichtenbelt

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Re: Electrical scheme of the output pin

Post by Benj »

Hi Jan,

In the microchip datasheets there is a section named electrical characteristics which should detail everything you need to know. If you need a good PDF viewer then I can highly recommend PDF-XChange, I've used it for years without a single issue and it's literally miles better than Adobe or the web browser defaults.

We add 220 Ohm to the signals on our E-blocks to ensure that nothing is damaged if a high output pin is shorted to a low output pin.

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Re: Electrical scheme of the output pin

Post by Jan Lichtenbelt »

Dear Ben,

Thanks for your reaction. I do not found in the specifications. But I found it in Wikipedia under TTL.
Totem-pole_uitgang.jpg
Totem-pole_uitgang.jpg (21.27 KiB) Viewed 6329 times
With 2 outputs, one high and one low, connected external with a 330 Ohm resistance I found R1 to be about 72 Ohm and the voltage Vce over the T1 to be 2V with a current of 12 mA. (PIC16F876A used)

I never realised that ouput high will not be 5V and output low will not be 0V. In the case above they will be respectively 4 and 0.2 Volt.

With kind regards

Jan Lichtenbelt

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Re: Electrical scheme of the output pin

Post by Benj »

Hi Jan,

No you should get pretty much 5V for an output high and 0V for an output low, depending on how much current is being drawn.

This is the circuitry of an I/O pin in the 16F876A.
PinDiag.jpg
PinDiag.jpg (36.36 KiB) Viewed 6306 times
FETs are used as they don't have the inherent voltage drops that transistors have. Instead they have a resistance but this will be very low, probably only a fraction of an Ohm.

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