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SN74CB3T3257: Logic level compatability

lakshmi pritha ramasamy
Prodigy 160 points
Part Number: SN74CB3T3257
Other Parts Discussed in Thread: TS3A24157, SN74AVC1T45

Hi,

We are using SN74CB3T3257PWR device. This is powered by 3.3V but the IOs we are providing is at 1.8V logic. Please share confirmation that this device supports 1.8V IO signaling with out any logic level translation in IO signal.

Because the VIH given for VCC= 3.3V is mentioned as 2 V. but our IOs are 1.8V signal which does not meet VIH of 2V. Please confirm that this device supports our 1.8V application.

Expecting your early reply.

Regards,

Lakshmi

  • 0
    TI__Mastermind 45340 points

    Lakshimi,

    The SN74CB3T3257 device supplied with Vcc = 3.3 V will pass 1.8V volts through the I/O pins 1A, 2A, ...4B2 without translation.

     

    The minimum voltage on the digital control signal pins EN and S for the SN74CB3T3257 device to recognize a logic high is Vih = 2 V when Vcc = 3.3 V.

     

    The Vih and Vil levels in the datasheet only apply to the digital logic pins S and EN not the I/O paths.

    Here is an app note explaining the CB3T family of devices for more information 

    Do you need help finding a switch without level shifting so that this question is not an issue?  What are you trying to do with the switch in your system?

    Adam

  • 0 in reply to Adam Torma
    Prodigy 160 points

    Hi Adam,

    Thank you for your early reply. Your answer clarified our doubt.

    It will be helpful if you can suggest a switch which supports 1.8V IO signal with VCC of 1.8V. This mux is for UART application as shown below.

    Regards,

    Lakshmi

  • 0 in reply to lakshmi pritha ramasamy
    TI__Mastermind 45340 points

    Lakshimi,

    I searched TI.com for 2 channel 1:2 SPDT devices that can operate from 1.8V supply.  All of these devices will pass 1.8V through the I/O path and have plenty of bandwidth for UART signals.  Let me know if you like any of these devices like the TS3A23159. 

    Thank you,

    Adam

  • 0 in reply to Adam Torma
    Prodigy 160 points
    Hi Adam,
    Thank you for sharing suitable part.
    As we are in final stage of design, we would like to proceed with SN74CB3T3257PWR as its IO port supports 1.8V UART signal while the device is powered with VCC of 3.3V. Your final confirmation helps us.

    Regards,
    Lakshmi
  • 0 in reply to lakshmi pritha ramasamy
    Prodigy 160 points
    Hi Adam,

    Awaiting for your confirmation to use SN74CB3T3257PWR device for UART application. Your early reply helps us.

    Regards,
    Lakshmi
  • 0 in reply to lakshmi pritha ramasamy
    TI__Mastermind 45340 points

    Lakshmi,

    If you are using the SN74CB3T3257 in the application below it will not work because it violates the recommended supply voltage and the input logic high min threshold.  If you change Vcc to 3.3V this device will still not work because the logic threshold Vih is still 2V. 

    There are several other devices mentioned in the previous posts that can operate from 1.8 V supply voltages.

    Thank you,

    Adam  

  • 0 in reply to Adam Torma
    Prodigy 160 points

    Hi Adam,

    Thank you for your reply. Yes we agree, with VCC =1.8V & Select and OE driven by 1.8V signals will not meet the recommendations. 

    Here we addressed the VCC, which is now 3.3V and we have used 74AVC1T45GW,125 (single bit level translator) for the select signals to convert it from 1.8V logic to 3.3V so that we can meet the VIH of 2V.

    Passing through IO signals (UART) are at 1.8V logic. Below is the actual implementation in our present design, hope now we are good to proceed, please confirm.

    Your early reply helps us.

    Regards,

    Lakshmi

  • 0 in reply to lakshmi pritha ramasamy
    TI__Mastermind 45340 points

    Lakshmi,

    I'm not sure what other information you are asking me to provide.  The SN74CB3T3257 recommended operating conditions states it can pass signals from 0 to 5.5 V, the supply voltage can be 2.3-3.6 V, and the logic control signals must be >2V for a valid logic high signal.  The system implementation you have above is within the recommended operating conditions in the datasheet as long as your UART signal bandwidth is less than <200MHz (typically I see UART <1MHZ.)

    If you would like a 2-channel 2:1 SPDT switch that can operate at Vcc= 3.3 V and accept 1.8V logic control signal take a look at these devices below.  Using one of these device will eliminate the need for the SN74AVC1T45 level shifter.  Below I used the TI.com selection tool to filter for 2-chanenl, 2:1 SPDT, Vcc = 3.3 V, and Vih <1.8 V.  It returned 8 results like the TS3A24157.   

    Let me know if you need something else.

    Adam

  • 0 in reply to Adam Torma
    Prodigy 160 points
    Hi Adam,

    Thank you for your reply. We will look in to the devices you suggest for our next requirements. Now we are proceeding with SN74CB3T3257 device as the datasheet recommended conditions are met in our application

    Regards,
    Lakshmi