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TMS320F28335: Enter SCI interrupt when pull down RX pin to GND without any data receivered

Vivian(MCU) Zhang
Intellectual 1545 points
Part Number: TMS320F28335


Hi,

Recently my customer reported an issue related to TMS320F28335 SCI module.

The customer pull down RX & TX pin to GND by 10KΩ resistance. After power on and enable SCI interrupt, they found that system will immediately enter SCI interrupt ISR,  but actually there's no data transmit on RX & TX. 

They test the RX & TX pin , voltage is 1.1V when add on an 10kΩ resistance， and try to change to 2kΩ, voltage drop down to 0.25V, the result is the same.

If remove the resistance, everything goes well as expected.

We know that RX& TX is not recommend to add external pull down resistance as there's pull up inside.

But we only curiosity about why the system will enter SCI ISR when there's no data transmit, no start and stop signal?

Here's the wave of RX pin. Green is an indicated IO, BLE is RX pin. RX keep the same voltage since power on.

Could you help comments ? Thanks!

Vivian

  • 0
    TI__Mastermind 42195 points

    Vivian,

    I have not confirmed it on a device, however my expectation is that you receive a framing error(maybe a break detect too) and an ISR trigger at the same time.

    Explanation:

    A start bit is simply the line being pulled low for some amount of time, since you are using a pull down of course this is true. After seeing a start of frame the SCI is going to receive all zeros until an error occurs due to a missing stop bit. The part I haven't tested if is an RX interrupt will occur at that time.

    Checking if the SCI sends an interrupt when a Stop bit is missed should be an easy check for the customer.

    Let me know your results!

    Regards,
    Cody