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interface TM4c123gh6pm (launchpad) with ADS1298

happyman
Prodigy 100 points
Other Parts Discussed in Thread: TM4C123GH6PM, ADS1298

Please guide me on, how do i interface TM4c123gh6pm (launchpad) with ADS1298.

I am stuck at the logic level translation part.

Referring to document Application Report SCEA035A - June 2004 Selecting the Right Level-Translation Solution, Section 1, point 1:

At a Vdd of 3.3V, applied to both ads1298 (receiver) and tm4c (Driver),

the Point1 : The VOH of the driver must be greater than the VIH of the receiver , Fails.

because TM4c (VOH -max) 2.4 volts  is less than ADs1298  2.904 (VIH-min) volts.

One possible solution that I can think of is:

=>  Reduce Vdd for ADS1298 to 2.6-2.7 volts while keeping TM4c Vdd at 3.3 volts.

But i am not sure if this would be a good approach as even a 1 volt variation in Vdd would make logic levels incompatible.

Please let know the right way to interface the 2 ICs.

thanks.

  • 0
    TI__Guru**** 244400 points
    Hello HappyMan,

    The VOHmin of TM4C is 2.4V and not VOHmax. Please double check the data sheet. Secondly the VOHmin value of 2.4V is a value when the current drain from the IO is large enough e.g. >50mA which normally a driver-receiver circuit will not do. Even if it does then adding a buffer on the path prevents loading of the driver pin.

    In summary, there is no problem as such from electrical perspective to interface TM4C and ADS1298.

    Regards
    Amit
  • 0
    Guru 117855 points

    happyman said:

    because TM4c (VOH -max) 2.4 volts  is less than ADs1298  2.904 (VIH-min) volts. 

    My reaction upon reading your post was much as Amit's.    Quote above summarizes your misunderstanding - but "how" could that have resulted?

    a) should that "VOH-max" (really) have been listed as 2V4 (max) I'd bet that the MCU manual provided performance data at different (i.e. reduced) Vdd levels.   If Vdd was limited to at/near 2V4 - then the result you report appears valid.    And - if such is the case - it's likely that you "missed" the MCU spec listing when Vdd was at/near 3V3!

    b) your report of the other device's "VIH-min" also registers as, "unusual."    (higher than I'd expect)

    Again - as Amit notes - when both devices are powered from the same 3V3 (adequate) source - I believe the results (after code debug/verify) will yield a, "happyman."   (though - based upon your appearances here - "questioningman" may prove more appropriate.)

  • 0 in reply to cb1_mobile
    TI__Guru**** 244400 points
    Hello cb1

    Indeed ADS1298 is listed as 0.8VDD as VIHmin. Thats 2.64V @ VDD of 3.3V. Must be some error made in calculations and observations by the poster.

    Regards
    Amit
  • 0 in reply to Amit Ashara
    Guru 47900 points
    Hi Amit,

    Back embedded now - and that (higher) number was sure to fall outside of what you/I believed to be normal/customary.

    I've complied w/your directive of yesterday as promised. Be well.
  • 0 in reply to Amit Ashara
    Prodigy 100 points

    I am really thankful for the back to back responses for my query. There were indeed 2 mistakes :

    1) I wrote "VOH -max" instead of "VOH-min" in the following line of original query:

    TM4c (VOH -max) 2.4 volts  is less than ADs1298  2.904 (VIH-min) volts.

    2) I wrote 2.9 volts instead of 2.6 volts because i used incorrect multiplier value 0.88 instead of 0.8.

    Actually the i could not locate the operating Vdd value for TM4c in Datasheet  Table 24-6. Recommended GPIO Pad Operating Conditions.

    So I had to assume Vdd to be 3.3v referring to , Table 24-5. Recommended DC Operating Condition where nominal value is written as 3.3v.

    Had the TM4c spec been specific like ads1298 wherein they clearly mention the multiplier (0.8) , perhaps things would have been easier for me.

    but even after considering the 2 corrections, the VOH-min for TM4c [2.4 volts] is still less than VIH-min of ADS1298 [2.64V].

    Can i safely conclude (subject to code debug/verify ) that this [2.64 - 2.4 = 2.4 v]  should not be a concern?

     

     

  • 0 in reply to happyman
    Guru 47900 points

    happyman said:
    Can i safely conclude (subject to code debug/verify ) that this [2.64 - 2.4 = 2.4 v]

    It would appear - that still - your math & its interpretation - defy any "safe" conclusion.

    As Amit earlier stated - that 2.4V is likely (only) to result when the pin is (heavily) loaded - not the case in your application.   (and that's a "safe" conclusion)

  • 0 in reply to happyman
    TI__Guru**** 244400 points
    Hello HappyMan,

    Ouch. TM4C129 data sheet is a huge data manual. Having to list the on top of every page the VDD, would open possibility to a lot more data conditions, and figuring the actual parameters for any system design make the end user;s life miserable.

    That is why "careful" read of the data sheet and "maths" is important before jumping to conclusion. May sound harsh (my apologies in advance) but not two devices are alike and cannot and must not be weighed in the same scale.

    Regards
    Amit
  • 0 in reply to Amit Ashara
    Prodigy 100 points
    That's fine Amit. Thanks for your inputs.