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SN74LS148N 8 bit priority encoder

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bayabi

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Hey there

I want to use 74LS148N model 8:3 encoder in my project. In theory and on isis it works well but I wonder if a certain valued capacitor or resistor should be connected to I/O pins. I am trying to read and use the data I obtained from the encoder by using 18f2550 as microprocessor. If you have any example circuit scheme or advice that u can share I will appreciate it.

Thanks in advance..
 

wp100

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Hi,

If both devices are operating a +5v ttl levels cannot see any reason to use additional caps or resistors apart from the standard 100nf across the power rails of each chip.

Also unless you are short of i/o pins cannot see the point in using a 74148 as the pic can readily do the conversion with software ??
 
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bayabi

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As you said I am short of pins that is why I must use 74148. So u advice me to just connect 100nf capacitors. By power rails do you mean Vcc and ground pins?

Moreover, 74148 has OH current of -0,4 mA and OL current of 8mA, however; 2550 has max current capability of 25mA on each pin. Thus,I guess I dont need to use any current buffer or anything?

Thank you for your fast help :)
 

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Hi,

Yes, each chip used should have the 100nf fitted as close to its vcc/ground pins as possible - standard practice.

No, no buffers etc needed, thats one of the whole points of TTL, its direct and compatible.
 
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bayabi

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Thank you very much my last question is even if it says on the datasheet "Voh typical value = 3,3 V" I keep getting 3,7 V output from the output pins of 74148 is it dangerous ? Moreover, do I need a pull up resistor or can I directly connect it to my 18f2550 PIC? On the 2550 s data sheet it says if input higher than 2 V it will serve as Input high..
 

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Hi,

If you read the datasheet in more detail you will see that Voh is specified under Test conditions where Vcc is at the minimum - depending on the datasheet typically around 4.5 or 4.75v so at a normal 5v Vcc, a Voh of 3.7v is fine.
Also the 3.7v may be a little low if your meter is draining too much out of the circuit.
Even if it gives out 5v thats correct and fine .

You do not need any pull ups etc, your Pic is fully compatible for direct input or output to most of the TTL family of chips.

WithTTL anything 0.8V and below is classed as a Zero and anything from 2.4V to 5.0v is classed as a One.

The thing you must be careful of on the Pic is that some if its input pins default to Analogue inputs at power on, so your code must set them to Digital Inputs otherwise you will get stange problems.
 

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