SN65HVD1794: About SN65HVD1794 Dpin & R pin pull up resistor selection

Betty Guo

Part Number: SN65HVD1794

Hi team,

For SN65HVD1794, D pin & R pin, do they need external resistor to pull up to VCC?

Customer said they check the R pin voltage is 5V without pull up resistor. Have they been pulled up inside the device? are they open drain output?

We use 5V to power SN65HVD1794, Can we use resisitor pull up D & R to 3.3V like below picture show, because customer use 3.3V MCU in their applicaiton.

Can you share the SN65HVD1794 functional block diagram?

thanks a lot.

over 5 years ago

0 Max Robertson over 5 years ago

TI__Guru 62910 points

Hi Betty,

The R output uses a push-pull driver, so it does not require an external pull-up resistance. Unfortunately, though, this would mean that the high level output would track VCC (5V in this case rather than 3.3 V). In this case you may want to use a level shifter in order to step the output voltage down to something that can be supported by your MCU. A simple way to do this would be use a pull-up resistance on the MCU input and a series diode on the R output of the transceiver. The diode could be arranged to conduct when the R output is low and to block the voltage when the R output is at 5 V (allowing it to be pulled up to 3.3 V on the other side).

Regards,
Max

0 Betty Guo over 5 years ago in reply to Max Robertson

TI__Expert 7300 points

Hi Max,

thanks for your reply.

how about the D input type, is also not use external resistor?

0 Betty Guo over 5 years ago in reply to Max Robertson

TI__Expert 7300 points

customer said that he check the D pin voltage is 5V when it is float. is it pull up to VCC inside the device?

0 Max Robertson over 5 years ago in reply to Betty Guo

TI__Guru 62910 points

Betty,

The D pin is a high impedance digital input that has an input current range of +/- 100 uA. Since the leakage current can be either polarity, you shouldn't rely on the input to be biased to a high level internally. To reliably have a logic-high or logic-low input the voltage should be biased externally.

Max

0 Betty Guo over 5 years ago in reply to Max Robertson

TI__Expert 7300 points

thanks Max

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SN65HVD1794: About SN65HVD1794 Dpin & R pin pull up resistor selection