Pull down resistor at the enable and address lines of MUX TMUX1208

Thread Starter

hoyyoth

Joined Mar 21, 2020
196
Dear Team,

I am using TMUX1208 in one of my Applications. To avoid any confusion during Power on I am planning to connect pull-down resistor to address
and enable line. The mux is controlled by an Arduino Nano.
Below is my circuit. May I know 10K is fine or not.

1636440508637.png

Regards
HARI
 

Thread Starter

hoyyoth

Joined Mar 21, 2020
196
Hi Irving,

To me, it is not required. But the team is afraid that when booting Arduino if something bad happens what to do.

Regards
HARI
 

Irving

Joined Jan 30, 2016
2,545
Arduino starts with all GPIO tri-state, so a pull-down on EN is all that is needed to ensure no output is selected. But even if one was, what impact would that have as SDA is dont care unless there is an SCLK signal. Incidentally why are you using an analog mux/demux as an SDA demux?
 

Irving

Joined Jan 30, 2016
2,545
Ah so SDA isn't related to I2C? I thought this was being used as an I2C expander.

So the BJT is switched on when its base is selected by the mux and the mux D input is actively driven high? All you need is a pull-down on the base of the BJT then at start up all these will be tri-state and as this is an analog mux the output will reflect the state of the D-input which will presumably tri-state and therefore no base current of any significance will flow.
 

Irving

Joined Jan 30, 2016
2,545
That seems complex... I guess there's a good reason for using relays to mux the SDA data to/from the DUT.

The relay is only active as long as the mux address, EN and D inputs to the TMUX1208 remain active & static. If they are all driven from the MCU then there won't be any base current to enable a relay during start-up as all are tristate. However with a 40mA pull-in on the relay you need a maximum of 200uA base current to turn the relay on and with a 680ohm base resistor that means only about 0.85 - 0.9v on the MUX output. I'd increase the base resistor to give yourself a bit more immunity from noise. Assuming the MUX D input is driven by a 3.3v logic signal I'd aim for say 2.7v giving a base resistor of 6k8 or 7k5.

Any reason you've chosen power relays rather than signal relays?
 

BobaMosfet

Joined Jul 1, 2009
1,951
Dear Team,

I am using TMUX1208 in one of my Applications. To avoid any confusion during Power on I am planning to connect pull-down resistor to address
and enable line. The mux is controlled by an Arduino Nano.
Below is my circuit. May I know 10K is fine or not.

View attachment 252213

Regards
HARI
What you're asking for is _knowledge_. Rather than give you a yes or no, you need a how & why. Here you go-

Pull-ups and pull-downs are used to control flow of current. It is the flow of current that determines STRENGTH of a signal on any line. Voltage determines direction of current flow, and intensity (how badly it wants to move), but not strength.

The entire concept of a pulling resistor is to gently hold a signal in one direction, unless over-ridden by a stronger current. As such, that should give you the why. A pulling resistor must use enough current to get the job done, but less current than an overriding signal.

4.7K-Ohm resistors are commonly used in 5V systems because that equates to about 1mA of pull. Which is adequate to pull a signal in a direction while at the same time being somewhat immune to noise, yet weak enough that a stronger current can override it, like 5mA.

This is based on Ohm's Law:

E = IR

Volts = Current * Resistance

If we switch that around for current, we see:

I = E/R
I = 5/4700
I = 0.00106A (or 1.06mA)

So if you are using a 10K-Ohm resistor, the current the pull-down is drawing (it acts as a mini-load on a signal not otherwise charged), is:

I = E/R
I = 5/10000
I = 0.0005A (or 500uA or 0.5mA - half a miliamp)

Read the datasheet to learn more about the limits of your pins, and IC total, which will help you determine how much energy you want to run through the device in off & on conditions.
 

Irving

Joined Jan 30, 2016
2,545
May I know how you are defining a small signal relay
let me explore the parts suggested by you...
thank you
Main difference is the contacts are intended for switching low-level signals. The G5 series are 3 - 5A contacts whereas a signal relay is usually 1A max and are often spec'd with a minimum signal level, eg 10mV/10uA. See this snippet from the Kemet spec, see the sections relating to low current usage...

1636477374470.png
 
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