Reducing the Voltage Level of TX line of an FTDI using voltage divider

Thread Starter

fieryfire

Joined Feb 14, 2017
129
Hello All,

I am trying to simulate the VIH and VOH limits of a micro controller which accepts 5V FTDI with upper and lower limits of VOH at 5 and 3V.
I am trying to simulate the exact voltage the MCU will stop communicated. For this purpose. Can i introduce a voltage divider of 1K and 1.5K from my TX line (5V) to step down the voltage to my MCU so that the MCU RX sees 3V instead of 5V.
Is there any danger to my MCU (msc1202y3 = 5V) circuit if i do this?.

Also Will the MCU see enough threshold current from the 1k , 1.5k voltage divided?

1595335359078.png
 

Irving

Joined Jan 30, 2016
1,070
You don't need a simulation for this, just some simple math!

5v -> 3v
Vout = Vin * R2/(R1 + R2) typical values R1 = 1k, R2 = 2.2k Vout = 5 * 2.2/(1 + 2.2) = 3.3v

If you look at FTDI spec it will typically say Voh (high output voltage) at some current, say 5mA, will be 4v, so output High, worst case will be 2.7v.

For the input, the 3.3v MCU needs to see Vinh(min) (Volts in high, min), typically 2.2v on a 3.3v system (66%), so there's plenty of margin, high side. The input of the MCU is voltage driven, most inputs are <1uA current these date (mosfet gate)., so 10k, 22k would work just as well.

Going from 3.3v output to 5v input is slightly different, but the Vinh(min) of a 5v system is usually < Voh of a 3.3v system, so all is well.
 

Thread Starter

fieryfire

Joined Feb 14, 2017
129
Hello Irving, thank you for the response
The MCU is also 5V, but its limits are between 3V and 5V for VIH., so i want to bring the 5V from my FTDI down to 3 or 2.9V to see at which volt the MCU stops communicating back. Since the RX of the MCU would receive at 2.9V when its VIH is 3V.

The key point you mentioned is the "The input of the MCU is voltage driven, most inputs are <1uA current these date (mosfet gate)., so 10k, 22k would work just as well. "

which makes perfect sense so that would mean, my 1k 1.5k divider could work to get the 3V right?
 

Irving

Joined Jan 30, 2016
1,070
The MCU is also 5V, but its limits are between 3V and 5V for VIH., so i want to bring the 5V from my FTDI down to 3 or 2.9V to see at which volt the MCU stops communicating back. Since the RX of the MCU would receive at 2.9V when its VIH is 3V.
For what purpose?

What will happen is it will get more and more receive errors and then go deaf... Why do you need to know actual level?
 

andrewmm

Joined Feb 25, 2011
668
Hello Irving, thank you for the response
The MCU is also 5V, but its limits are between 3V and 5V for VIH., so i want to bring the 5V from my FTDI down to 3 or 2.9V to see at which volt the MCU stops communicating back. Since the RX of the MCU would receive at 2.9V when its VIH is 3V.

The key point you mentioned is the "The input of the MCU is voltage driven, most inputs are <1uA current these date (mosfet gate)., so 10k, 22k would work just as well. "

which makes perfect sense so that would mean, my 1k 1.5k divider could work to get the 3V right?
You might be confusing simulation and reality.

The data sheet has Voh and Vol at a given mA,

BUT

That is guaranteed over process, PSU voltage and temperature variations.
At mean PSU voltage, at standard room temperature, and with a random part, you will get much better VoH / VoL numbers.

As such , what your doing is rather unusual, and I'm wondering what are you expecting to gain by measuring / simulating the voltage outside the data sheet limits ?

This might be of interest

https://learn.sparkfun.com/tutorials/logic-levels/all
 

Thread Starter

fieryfire

Joined Feb 14, 2017
129
You might be confusing simulation and reality.

The data sheet has Voh and Vol at a given mA,

BUT

That is guaranteed over process, PSU voltage and temperature variations.
At mean PSU voltage, at standard room temperature, and with a random part, you will get much better VoH / VoL numbers.

As such , what your doing is rather unusual, and I'm wondering what are you expecting to gain by measuring / simulating the voltage outside the data sheet limits ?

This might be of interest

https://learn.sparkfun.com/tutorials/logic-levels/all
Thank you Irvingand Andrew for the response.

My intention is to recreate a failure that we see in my device. We use our setup at high temperatures. and we suspect that VIH threshold values increase with temperature.

My plan is to increase the temperature of my device. and artificially reduce the TX voltage to my device(MCU) using a potentiometer and play around the margin of about 3V to see where it fails.
 

andrewmm

Joined Feb 25, 2011
668
I assume 180 faranheight,

Are you using industrial grade parts ?

The ViH and L will change over temperature , voltage and process , but will be within the data sheet specification.

If its logic to logic, and your within the data sheet bounds, this should not be a problem,

Where I have seen this, is when people try to use voltage dividers to go between logic series , or when people have designed by try it and see, one prototype worked, so they went into production.

The thresholds will vary, the drive currents and speeds will vary over PVT, one just has to design to data sheets
 

Thread Starter

fieryfire

Joined Feb 14, 2017
129
I assume 180 faranheight,

Are you using industrial grade parts ?

The ViH and L will change over temperature , voltage and process , but will be within the data sheet specification.

If its logic to logic, and your within the data sheet bounds, this should not be a problem,

Where I have seen this, is when people try to use voltage dividers to go between logic series , or when people have designed by try it and see, one prototype worked, so they went into production.

The thresholds will vary, the drive currents and speeds will vary over PVT, one just has to design to data sheets
180Celcius,

Yes they are industrial grade parts specifically made for high temperature

The example you gave us exactly concerns the product thats being used. We had 20 systems that were ok. one of them failed. and we suspect that this is the cause of the failure.

The customer is indeed using it outside the range. its supposed to be rated for 150C but they need it until 175. Also the customer uses a 3.3v ftdi to communicate when the mcu needs a 5V signal for communication. Since the voltage thresholds is between 3v and 5v. then the temerature has a very high influence since they can come close to this threshold limits.

for ex. a 3.3v to our system at 175 C would not until bring our VIH up but also the VOH from their system down. At one temperature the point meets and causes a failure
 

Irving

Joined Jan 30, 2016
1,070
When i worked on mil grade equipment we wouldn't design around those extremes. Within 20degC of the limits hot or cold we had to build in other approaches, such as active insulation (most mil equipment you can't use open cooling for EMR reasons) or active heating.

Why/where would you take such systems to those temperatures?

Sounds to me you need to revisit how you cool/insulate or bin select parts prior to manufacture.
 

andrewmm

Joined Feb 25, 2011
668
Silicon stops working above 125 Deg C ,

I did some mill stuff that had to work at top of a funnel, in the fumes, at around 180 to 200 C, we had to use a cooling system to keep the chips below 125. And they were screened parts for the application. ( The chips dissipated a watt or two, the cooling a few hundred watts )

had the same sort of problems in a down a boor hole application,

The FTDI chip, is that rated for this sort of temperature ? If you are using one with a charge pump on it, then they are temperature sensitive. The capacitors just degrade when that hot.


I would be very worried. If you test say two units out side limits that work, then make another 98 without testing outside limits, what confidence do you have that the 98 will work ? I'd say zero.

Even if you test every unit to its limits, running the silicon above 125 , degrades the chip with time. To a first level, the doping diffuses.

I hope this is not for a critical / mission sensitive design.
 

Irving

Joined Jan 30, 2016
1,070
I hope this is not for a critical / mission sensitive design.
I concur. The radio bay of a certain plane, engine shut down, on an artic airstrip can get down to -60C with wind chill. The same bay, stuck on top of the engine (yes, wasn't my ideal place) on a desert airstrip in full sun with the engines running can hit 170C. To do that in the same bit of equipment takes some creativity... and it doesn't involve exposing the chips to those extremes...it just would be madness.
 
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