TCAN1044V-Q1: STB, TXD pull up

1. The datasheet says "pull-up", not "pull-up resistor". As shown in the block diagram, the pull-up is a pull-up current source.

2. The STB pin can source up to 20 µA when pulled low (see its I_IL specification). You can add an external resistor if you want more pull-up current for reduced noise sensitivity.

The TXD pin also has an I_IL specification.

The size of the external pull-up resistor would depend on the requirements of your circuit. A common value is 10 kΩ.

Zena,

Clemens's comments are correct. This is usually a consideration when there is a weak driving source for TXD/STB or when the driving source is open-drain only. Based on your customer's implementation it may or may not be required. It will not hurt to add an additional pull-up resistor as mentioned.

Best,

Danny

Most CAN controllers drive the TXD signal with a push/pull output and therefore do not need a pull-up resistor. (You would configure the CAN TX output as open-drain only when you need to do level shifting, or to connect multiple transceivers.)

Hi Zena,

An active MCU will most likely drive the STB pin with a push/pull output. The reason why this pin has an internal bias (and often an external one) is so that the input pin remains in a known state when the MCU pin is high, often the case before the software has configured its IO. Because of this case, all of the input pins internal biases will put the transceiver into a state that will be least impactful to the system - recessive input to TXD and low power modes for mode control pins. 

The reason for including external biasing resistor is to override the integrated bias if a different behavior is desired or to increase the signal's noise immunity by providing a stronger bias that will not be as easily modified by nearby coupled signals. 

Let me know if you have any more questions. 

Regards, 
Eric Schott