If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

# TCAN4551-Q1: Cancel weak pull up at SCLK pin

Part Number: TCAN4551-Q1

Hello team,

According to the datasheet, there is weak pull up on the SCLK pin.
However, I want SCLK to be LOW, when CPU start up.

What is the resistor value I need to pull down in order to cancel the weak pull up?

Best Regards,
Kei Kuwahara

• Kei,

The answer somewhat depends on your VIO voltage in your application, because the thresholds are based upon VIO.

The strongest possible pull-up from SCK is given as IIL ≤ 100 µA across process, voltage, and temperature. We would want a resistor that can keep the SCK voltage below VIL = 0.3*VIO. If we assume a VIO = 3.3 V, then this would be a low-level input threshold voltage of 0.99 V ≈ 1 V.

We can use V = IR as VIL ≤ IILR

R ≤ VIL / IIL

R ≤ (1 V) / (100 µA)

R ≤ 10 kΩ

So, I would say less than 10 kΩ would be strong enough. To determine the maximum strength, this would depend on your SCK source.

Best,

Danny

• Hi Danny-san,

Thank you for the detailed answer!

Best Regards,
Kei Kuwahara