FreeLists / si-list / [SI-LIST] Re: How to select the pullup/pulldwon resistor

["Henrik G. Madsen" <hgm@xxxxxxxxxxx>]

Hey Peter

For unused inputs the issue is simple and a matter of pure math and how =
much noise margin you would like to have.

If you look at a datasheet for a SN74LS74A, it specify the maximum input =
leakage current in high and low state

Iih =3D 40uA (Vi=3D2.7V)=20
Iil =3D -0.8mA (Vi=3D0.4V)

If the minimum supply voltage is 4.5V (5V +-10%) and you want a noise =
margin of 400mV (standard for TTL), then the pullup and pulldown =
resistor values are giving by.

Rpullup <=3D (4.5V -(2.0V +0.4V))/ 40uA =3D 52.5 k ohm

Rpulldown <=3D (0.8V-0.4V)/ 0.8mA =3D 0.5 k ohm

The huge difference in requirement to the pullup and pulldown is caused =
by the difference in leakage current and the distance from Vih/Vil to =
the supply voltages.

In modern CMOS electronic, the leakage current is lower and the Vih/Vil =
are placed more symmitrically between power supplies.

On top of that you often wnat to reduce the number of different types of =
components as much as possible to reduce production cost.

Hence many schematics will only contain a couple of different resistor =
values, hence a simple pullup might be a 2.77k +- 0.1% because the size =
and precission is necessary somewhere else in the schematic.

Regards
/Henrik

-----Original Message-----
From: peter zhu [mailto:peter.zhu@xxxxxxxxxx]
Sent: 28. november 2002 12:47
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] How to select the pullup/pulldwon resistor


Hi all:
In hardware design, we often encounter the problem of pullup or pulldown =
resistor, i very puzzled about it:
1.how to determine pullup or pulldown.
2.how to determine the value of pullup or pulldown resistor.
These are two common hardware design problem. I hope can see some =
detailed materials about it. Help me!

Regards!

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