TJA1051T/3 internal pull down resistor on pin S

[nexy_sm]

Hi all,

I am currently dealing with the CAN transceiver TJA1051T/3. It has pin named S which is used to bring the transceiver in Normal or Silent mode. I am not sure if I am correct but I think the transceiver should stay Silent until the MCU activates it by setting pin S low. However, S pin has internal pull down resistor which brings it in Normal mode by default. Since the MCU I am using has no pull up or pull down enabled in the first place, it will allow internal pull down to dominate.

My question is what is the value of the internal pull down resistor inside the S pin of the TJA1051T/3?

[xiangjun_rong]

Hi, Savic,

Generally, the pull-down resistor is about 20K to 50K ohm. You can connect the pin with push-pull GPIO pin of mcu to the S or EN pins of the TJA1051T/3.

Hope it can help you

BR

XiangJun Rong

[nexy_sm]

Hi Rong,

there will be a short glitch between the POR and pin configuration during which the CAN transceiver is enabled. This is something I would like to avoid although it is not that critical in my opinion. I also wonder why they included internal pull downs instead of pull ups.

Regards

[xiangjun_rong]

Hi, Nemanja,

Regarding the question why they included internal pull downs instead of pull ups, assume the scenario that the processor connected to the CAN transceiver TJA1051T/3 is power off, but the TJA1051T/3 is powered up, because the CAN bus is network connection, in the case, EN is low, S is low, the CAN driver pin CAN_H and CAN_L are float, so the node with TJA1051T/3 does not take effect on the CAN bus, in other words, the CAN bus can work although the node of TJA1051T/3 does not work.

Hope it can help you

BR

Xiangjun Rong

[nexy_sm]

In my case, I use TJA1051T/3, so EN pin is internally connected with Vcc. This means as far as I could understand that, in order to go silent S needs to be high, but it is internally pulled to low.

[xiangjun_rong]

Hi, Savic,

If you want to control TJA1051T/3 to enable or disable even silent/ normal mode, you can connect the MCU GPIO pins to the EN/S pins. If you do not want to control them, you can connect external pull-up resistor for example1K ohm to the EN or S pin based on your requirement.

Hope it can help you

BR

Xiangjun Rong