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I'm going through the Datasheet of DRV8860 and got confused by the pull-up setting of DOUT. In the Datasheet the block Diagram shows the pull-up as follows:

enter image description here

In the "Electrical characteristics" table it is stated that the pull-up resistance is typically 1.4k Ohm and connected to an 5.7V internal voltage.

enter image description here

So my questions: What's the reasoning behind this configuration? Why for example they would not put a high value ("weak") pull-up there instead? Then the user could still oversteer it with a lower value pull-up resistor. But with this configuration, the voltage is pretty much fixed to 5.7V.

Or am I missing something?

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  • \$\begingroup\$ Above link is Rev. E from Ti Website. ti.com/product/DRV8860#design-development - where did ou find a newer one? \$\endgroup\$ Apr 6, 2021 at 15:42
  • \$\begingroup\$ I could have sworn the link in your post was a link to older datasheet on Mouser or some other distributor. It might have been that I myself clicked on an older datasheet first and after that on the TI page. I am sorry for the confusion. I will try to find the other datasheet, which used different wording. \$\endgroup\$
    – Justme
    Apr 6, 2021 at 16:20

2 Answers 2

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Why for example they would not put a high value ("weak") pull-up there instead?

Well, the main use of DOUT is when daisy-chaining several DRV8860 devices: -

enter image description here

And, when used like the above, a relatively low value for the internal pull-up resistor is needed to ensure that data can be transferred daisy-chain fashion at reasonable speeds. Of course, TI could have used a much higher value internal resistor and then, when daisy-chaining, suggest that an external pull-up is used.

But, it's probably a committee/marketing thing where votes were taken around a table and the outcome was a compromise. If you really do need feedback of the serial data you have dropped into the chip, then you would be advised to use a potential divider from DOUT to match the Vcc of your host.

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  • \$\begingroup\$ So it's basically, they favored "daisy chaining without additional parts at full speed" over "being able to use the open drain with different voltage levels". But the 5.7V are still odd, as in worst case you are even or of the VDD+0.6V range for some 5V MCU inputs. \$\endgroup\$ Apr 6, 2021 at 11:37
  • \$\begingroup\$ @KarlKarlsom that's my interpretation. Yes 5.7 volts is odd and just a little beyond 5 volts to make it uncomfortable. I expect they had performance problems in the chip testing and decided that they could cure it with a higher-than-5-volt internal rail. \$\endgroup\$
    – Andy aka
    Apr 6, 2021 at 11:42
  • \$\begingroup\$ @Justme can you point me to this newer datasheet & chapter? \$\endgroup\$ Apr 6, 2021 at 15:49
  • \$\begingroup\$ RevE appears to be the latest on the TI site and that's the one linked in the question. \$\endgroup\$
    – Andy aka
    Apr 6, 2021 at 15:54
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The internal pull-up resistances are meant for not having those pins floating under any conditions: bad written software, for example.

The value of 1.4 kOhm is tagged as "typical" because the manufacturer doesn't or can't test it in production.

It's not that easy to understand why TI, the manufacturer, didn't set a higher pull-up resistance value.

As far as I know, pull-up resistors are actually manufactured as p or n MOSFET's appropriately biased. That means that it's not easy to control the resistance value.

The voltage of those input signals can be overridden by the voltage stated/asserted by your micro controller's output pins.

Micro controller's output pins have a typical output resistance lower than 10 Ohm, therefore 1.4 kOhm doesn't really count.

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  • \$\begingroup\$ Regarding the last part of your answer: Dout is an output of the DRV and would typically go to an input. \$\endgroup\$ Apr 6, 2021 at 9:48
  • \$\begingroup\$ The microcontrollers I have used the past 15 years or so since their introduction do have true resistors for pull-ups and pull-downs, with a switchable PMOS/NMOS. Besides SDATOUT is an output, and the datasheet refers the pin as both being opendrain with pullup or push-pull depending on which page you read. Yes, it is simply used for low-speed daisy chaining without external components. \$\endgroup\$
    – Justme
    Apr 6, 2021 at 12:33

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