The abnormal output in the test of SJA1124AHG terminate resistor

[xuelong]

Hi NXP expert,

We did a test to verify the pull up resistor of SJA1124AHG. The test steps are as follows:

1. The test setup is showed in the attached picture, Rtest = 1Kohm;

2. Turn off SW1(disconnect Rtest), get the recessive signal voltage V1;

3, Turn on SW1(connect Rtest), get the recessive signal voltage V2;

4, The pull up resistor of SJA1124AHG will be calculated: Rterminate = Rtest (V1/V2–1).

However, in the test, when disconnected Rtest, SJA1124AHG has a normal output:

But when the Rtest is connected, no any output signal will be detected:

Does SJA1124AHG could not support 1K load? please help us to analysis this.

And this test method is implement on TJA1021, TJA1021 works well.

[TomasVaverka · Accepted solution]

Hi Chen,

The integrated LIN commander pull-up resistor in the SJA1124A is 1Kohm. However, the integrated LIN commander pull-up is protected against too long dominant times. See below snapshot from the data sheet.

For the LIN commander pull-up resistor value measurement, an external pull-down resistor should be chosen that keeps the LIN signal recessive. For instance, Rtest=2kohm will result in 66% Vbat, which is still LIN recessive.

BR, Tomas

[xuelong]

Got it, thank you so much, Tomas!