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SN74AXCH8T245: Voltage translation problem : SN74AXCH8T245

Mehmet Gunce Akkoyun
Prodigy 75 points
Part Number: SN74AXCH8T245
Other Parts Discussed in Thread: SN74AXC8T245

I designed a circuit using the Telit GE190C1-EUX GSM modem. Since the GSM side operates at a 1V8 voltage level and my processor operates at a 3V3 level, I used the SN74AXCH8T245 voltage level translator chip from Texas Instruments (as seen in the attached schematic). In data communication, the signal sent from the GSM side (HIGH) is converted to the 3V3 level side instantly. However, although the signal on the GSM side drops to LOW, this drop is not occurring on the processor side. As can be seen in the area marked with a red circle, although the PMon(1v8) signal drops to LOW, the PMon(3V3) signal does not drop to LOW (it drops from 3V3 to 3V0). However, when the SWR signal drops to LOW on the GSM side, it also drops to LOW on the processor side. Despite all software settings being the same and the hardware being identical, I am experiencing issues with the Ring and PMon signals. Could you please help me understand why this is happening?

  • 0
    TI__Mastermind 42976 points

    Hi Mehmet,

    Please note that the schematic is blurr. If I understand correctly, B is the input and A is the output for B to A communication. And, some A output ports go low when B is low, and others do not.

    Typically, such offsets can be caused if there is a voltage divider and would recommend to confirm the CPU or the communication line do not have internal pull-ups / pull-downs specific to the A ports not driving low. If the case, please remove. If not the case, replace with a known good unit to help further clarify if concerns are specific to the unit in question, thanks.

    Best Regards,

    Michael.

  • 0 in reply to Michael Ikwuyum
    Prodigy 75 points

    We are using the voltage translator SN74AXCH8T245, and upon examining its features, we notice that it has a 'bus-hold' characteristic. Our suspicions are that the issue we are experiencing is due to this feature. Could this be the reason?

  • 0 in reply to Mehmet Gunce Akkoyun
    Guru 307490 points

    Yes; bus-hold inputs should not be used with pull-up/-down resistors.

    Do you need bus hold? If not, just use the SN74AXC8T245.