MAX31850K & Maxim DS9490R 1wire to USB adapter

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houmr
 
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MAX31850K & Maxim DS9490R 1wire to USB adapter

Post by houmr »

Hello,

is it necessary to use a level shifter if Adafruit MAX31850K is connected to Maxim 1wire to USB adapter DS9490R?

Thank you
Zdenek

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adafruit_support_mike
 
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Re: MAX31850K & Maxim DS9490R 1wire to USB adapter

Post by adafruit_support_mike »

I don't know anything about the DS9490R, and wasn't able to find a datasheet that says what voltage it uses on its 1-wire data line.

The MAX31850 has an absolute upper limit of 4v for input voltage, so a level-shifter would probably be a good idea.

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jlast
 
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Re: MAX31850K & Maxim DS9490R 1wire to USB adapter

Post by jlast »

I am looking to do the same thing as @houmr, reading a thermocouple with a DS9490R USB 1-wire bus master. The DS9490R uses the D2490 chip. I found this in the DS2490 datasheet:
The DS2490 directly interfaces a USB port to a 1-Wire bus. As shown in Figure 1, the DS2490 incorporates a USB physical interface, a USB device controller coupled with a 1-Wire-specific USB function core, and a 1-Wire bus interface controller. The 1-Wire interface controller shapes the slopes of the 1-Wire waveforms, applies strong pullup to 5V, and reads the 1-Wire bus using a non-TTL threshold to maximize the noise margin for best performance on large 1-Wire Networks. 1-Wire waveform timing is accurately controlled with a crystal-based oscillator.
Also this:
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This means it is using 5V logic, right? So it would seem the level shifter is required for the MAX31850K breakout.

My question is, I still need a 3.3V supply for the LV side of the level shifter, which isn't available from the DS9490R. Can I take it from "3V3" on the MAX31850K board?

Thanks!

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adafruit_support_mike
 
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Re: MAX31850K & Maxim DS9490R 1wire to USB adapter

Post by adafruit_support_mike »

Whew.. this is one of those cases where vocabulary matters.

The term "5v logic" implies specific voltages for high and low values, which the DS9490R apparently doesn't obey. It does take 5v power from the upstream USB connection and uses that 5v as the pull-up voltage on the 1-Wire line, so the "need a level shifter" part is right even though the exact high/low logic voltages aren't.

You'd have to connect the MAX31850K in 'local power' mode (5v to the Vin pin) rather than 'parasite power' (5v on the data line), because the latter would kill the chip. If you do use local power, the onboard 3.3v regulator can easily supply 3.3v to a level shifter.

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jlast
 
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Re: MAX31850K & Maxim DS9490R 1wire to USB adapter

Post by jlast »

Mike,

Thanks. Indeed I am totally new to electronics so I'm just trying to make sense of this as I go along :) Thanks for the clarification.

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jlast
 
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Re: MAX31850K & Maxim DS9490R 1wire to USB adapter

Post by jlast »

Just thought I would post a pic of my finished 31850 board. No problems, does the trick. Interestingly, if I unhook the +5V power supply, it still works fine. Don't quite understand why, as the level shifter is no longer getting power on its HV side. Anyway, just posting as reference in case anyone else is wanting to do the same thing.
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adafruit_support_mike
 
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Re: MAX31850K & Maxim DS9490R 1wire to USB adapter

Post by adafruit_support_mike »

Nice looking build! Thanks for posting it. ;-)


WRT the 5v line, the only things connected to the level shifter's HV pin are pull-up resistors for the IO pins on that side.

The section of the DS9490R datasheet you quoted above says it "applies strong pullup to 5V", and that would be parallel to the ones between the IO pins and the HV pin. Since you have that, the HV pin on the level shifter isn't strictly necessary.

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