MKE04 UART0 TX on PTA3 pullup

[jaroslavbrazdil]

Hi,

I used Uart0 Tx signal on PTA3 pin on my design with MKE04Z8VFK4.

The first set of 15 pcs of the module works fine.

I have new set of 25pcs of same module, no one of them works.

The problem is with UART0 Tx signal. PTA3 pin is open drain, so I use Internal pullup resistor by PORT_PUEL register.

It works fine for the first 15pcs , the Tx signal (connected directly to SN65HVD485 driver) is clear, more then 4V high.

It does not  work for new set of modules. System ROM and SIM_SRSID is same for all MCUs.

So the question is, how the pullups works, is it possible tu use pullups for UART?

If not, why one set of MCU works (all of them, with Tx level over 4V) and another set not (no one works)?

System ROM and SIM_SRSID is same for all MCUs.

When I set the pullup, the Tx signal goes high (4,5V), when I enable the transmitter (set UART0_C2    TE)

the Tx signal drops to 0.

I can use external pullup, but I have 40 modules ready and external resistor is not exactly elegant.

[kerryzhou · Accepted solution]

Hi Jaroslav,

Thank you for your good question.

PTA3 and PTA2 in KE series is the special pin, you can find it from the KE04 reference manual.

1. PTA2 and PTA3 are true open-drain pins when operated as output

2. PORT_PUEL[PTAPE3]: This control field determines if the internal pullup device is enabled for the associated PTA pin. For port A pins that are configured as outputs or Hi-Z, this field has no effect.

If you use the PTA3 as the UART pin, this pin is the true opendrain, internal pullup is not working, you must add 4.7K to 10K external pullup.

The module works on your side with SN65HVD485 driver, I think it due to your external TI's 485 transceiver.

You will find your SN65HVD485 equivalent input already have the pullup, but this pullup normally very weak, so it may caused some of your board was working, some not.

So, for working stable, please must add the external pullup resistor.

Wish it helps you!

If you still have question, please let me know!

Have a great day,
Jingjing

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[jaroslavbrazdil]

Hi Jingjing,

thank You for the answer, 

I assumed, that it is as you wrote.

The function of pullup in datasheet was not clear enough for me.

Best regards

  Jaroslav