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Part Number: TXB0104
I am using TXB0104 for uart conversion from 1.8V to 3.2V. Initially the level conversion of constant high signal is happening. But when a data bit is sent, it starts to generate a series of sin waves as noise and settles after some 30 sec or 1 min. Please help me on this. I have attached the schematic of it for reference.
Note: No pull ups are there on UART lines. Do we need to put them?. On one end of TXB IC there is GSM controller and on other end its open to an external connector.
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In reply to Karan Kotadia:
We found that it is not getting enough input impedance, so we added series resistor of 470 ohms to TX and RX, it started working fine. Will the TXS series work without the series resistor to inputs, any suggestion on it please?
Aditya Teja Damera
In reply to Aditya Teja Damera:
Please send the scopeshots if available. The series resistors are added on the input or output lines? Series resistors provide impedance matching as well as suppress ringing/ overshoot/undershoot when interfacing into hi-z loads. What is the loading at the output? TXB devices are sensitive to cap loading and work up to 70pF cap loading as mentioned in the datasheet and the app note here :
TXS doesnt have series resistors internally and also for UART, we do not recommend using TXS for UART pushpull signals. If drive strength is a concern, then as Karan has suggested, the DIR controlled translation family could be used like the SN74AVC2T245 device(2 DIR pins for 2 channels enables RX and TX transmission) or the SN74AVC4T245( for 4 line UART).
Refer to the FAQs on common recommended translation devices for std interface.
Watch the Introduction to AXC Family
Read the app note on Power Sequencing for the AXC family
Evaluate the AXC8T245 EVM and watch the translation using the EVM
In reply to ShreyasRao:
The resistors were placed at Tx and Rx of uart lines on 3.3V side
PFA the scopeshots.
At 1.8V side
At 3.3V side
Please note, initially a when Rx is high, I am able to see the level shift happening, but when ever I am sending a data, thats when this nosie is observed.
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