how to make pin low or high?

Thread Starter

abc77

Joined May 10, 2017
59
In PCM1789 DAC, it instructs to make pin low or high depending on feature I want.

If I want to make a pin low, do i directly connect it to ground? or do I need to add pulldown resistor between ground and the pin? if resistor has to be added, how to know the value of the pulldown resistor?
 

Thread Starter

abc77

Joined May 10, 2017
59
hi,

thanks, one more thing, what is the difference between pulldown(pullup) and simply connecting to ground(or supply) as we see in electronics circuits?
 

crutschow

Joined Mar 14, 2008
23,742
hi,

thanks, one more thing, what is the difference between pulldown(pullup) and simply connecting to ground(or supply) as we see in electronics circuits?
A pulldown/pullup resistor is used to establish the logic state of an input that you may or may not have an external connection.
The external connection can then override the default logic state of the resistor.

If there's never going to be an external connection then you can tie it directly to the desired logic state.

In general you never want to leave inputs floating on either ICs or transistors.
 

kubeek

Joined Sep 20, 2005
5,647
On the other hand, if you are making a PCB it is better to use resistors if you have the space, because once you realize you goofed it and need to change the polarity you will have a point that is much easier to change than if you had it connected directly.
 

crutschow

Joined Mar 14, 2008
23,742
ok and how does one choose the pulldown(pullup) resistor value?
It just has to be small enough to carry whatever the input pin current is, which is typically quite low, and still maintain a logic low or high as needed.
10kΩ is a commonly used value that takes care of most inputs (except for the old bipolar TTL logic inputs which require a much smaller resistor for pulldown).
 

MrChips

Joined Oct 2, 2009
19,729
In order to determine the required pull-up or pull-down resistor value you need to look at the current and voltage requirements of the input pin as stated in the datasheet. This will give you the minimum and maximum values which usually happens to be a very wide range for low current devices.

The datasheet also states that the device is compatible with LVTTL signals.

As @crutschow suggests, 10kΩ is a good compromise for CMOS and LVTTL devices.
 

Papabravo

Joined Feb 24, 2006
12,693
The other reason not to use direct connection is that ATE (Automated Test Equipment) has a very hard time with direct connections. Testability was often an overridding concern.
 
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