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SN74LV244A vs SN74ALS244C-1

Weiren Tee
Expert 1100 points
Other Parts Discussed in Thread: SN74ALS244C-1, SN74LV244A, SN74LV244AT, SN74ALS244C, AM3354

Dear TI Engineers,

Need your advise on the followings :

1. The datasheet in the link below, page 3, the conditions for VIH and VIL are not given.

http://www.ti.com/lit/ds/symlink/sn74lv244a.pdf

2. May I know the Input differences between the SN74LV244A vs SN74ALS244C-1? In another words, what kind of inputs.

Thanks.

 

Regards,

Weiren

 

 

  • 0
    TI__Mastermind 46480 points

    The function is the same however since that are t different technologies their voltage and current spcs will be very different.  The ALS will have TTL inputs while the LV will have CMOS type inputs. If you need ttl input Levels you can use the SN74LV244AT instead. 

     

    The Vih and Vil Levela are in the datasheet on page 3.

     

  • 0 in reply to Chris Cockrill
    Expert 1100 points

    Hi Chris,

    Thanks for your reply. Could you also help to advise on item 1? The conditions are missing in the datasheet.

    Meanwhile, I have a very strange situation here.

    AM3354's SPI0_XX is interfaced to the input of the SN74ALS244C. All the SPI signals are working well except the SPIO_D0. The SPIO_D0 logic level is measured to be: '0'-> 0V, '1'->1.8V (We are expecting' 0'-> 0V, '1'->3.3V).

    When SN74ALS244C is removed, the logic signal become normal (0'-> 0V, '1'->3.3V).

    If we replaced the SN74ALS244C with NXP 74HCT244C, the operation becomes correct(0'-> 0V, '1'->3.3V).

    If we replaced the SN74ALS244C with SN74LV244A, the operation also becomes correct(0'-> 0V, '1'->3.3V).

    This observation is made over more than 30 prototypes build. Seems like the problem is with the SN74ALS244C.

    Could you share your thoughts on this case?

    Additional information : AM3354 SPI I/O is powered by 3.3V, the SN74ALS244C's VCC is 5V. Signal is captured at the AM3354's SPIO_D0 signal line to pin15 of the SN74ALS244C.

     

    Regards,

    Weiren

  • 0 in reply to Weiren Tee
    TI__Mastermind 46480 points

    Both LV and HCT are CMOS parts and ALS is Bipolar. It could be that the input levels are very different and Bipolar is very leaky compared to CMOS. I dont know what the cause could be without seeing scope shots of the inputs.

    They big differenc in Bipolar and CMOS is that the output for Bipolar will be Vcc-1 while CMOS is equal to Vcc.

     

     

  • 0 in reply to Chris Cockrill
    Expert 1100 points

    Hi Chris,

    Thank you very much for your feedback. Below is the signal we captured at the Signal input of the IC. From the signal captured, we can see that the logic '1' is only 1.8V. The weird thing is, only one channel(SPIO_D0) is affected. We will be rerouting this signal to another channel to see if the issue follows the SPIO_D0 from the AM3554 or not. If it does not follows, I think that this specific channel of the whole batch of ICs maybe faulty or there are something that is causing it to fail.

    Regards,

    Weiren