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TMS320F28379S: PWM High Output on Reset(/XRS)

Part Number: TMS320F28379S

Dear. Sir/Madam,

I am currently using F28379S 100pin Package,
and I have a question regarding the usage of 'Reset' Function.

When using POR Reset, the output from (PWM7A,B / PWM8A,B) maintains LOW Level when Power is both ON and OFF.
When I input(Maintain) Low on /XRS Pin, the output from (PWM 7/8) initially emits High and goes down to Low.
I have carried the test using a Tact Switch with R-C and C paralleled.

<Reference Manual>

According to 8.8 Internal Pullup Configuration Requirements:
- Pull-Up is disabled for Input Mode
- However for lesser Pin Packages, Pull-up is enabled for Input Mode

According to 3.3.1 Reset Source:
- IOs for POR and /XRS are Hi-Z
--> However, as I am using a lesser Pin Package, I presume that Pull-up would still be enabled in Input Mode.


1. When turning On/OFF the Power by POR, there is no PWM High Output.
But when using /XRS Low Input, the output initially emits High and goes down to Low. (Maintains Low afterwards)

Please let me know why there is a such difference in terms of Output between POR and /XRS Low Input.

2. I have tried using 10kOhm Resistor on PWM 7,8 for Pull-Down. But there is still a slight High Voltage being emitted.

Can you please let me know the specification of internal Pull-Up Resistance? - In kOhms would be enough for me.

*I have attached the images of the Output Waves for two cases - With / Without the Pull-Down Resistor

The graphs indicate following condition: The High output Wave when I input /XRS Low Input(Maintained)
--> You can see that the graphs show exactly the same symptoms that I have mentioned above.

No Pull-down

10k Pull-down

5k Pull-down

  • Hi Bae,

    Thanks for the information and scope shots. To answer your questions:

    1. The GPIO pullups are disabled upon reset, so if the GPIO is low, we would expect it to remain low on reset as well. 

    2. We do not have a datasheet specification for this resistance, but we do have the pullup current found in the datasheet table 6.6 Electrical Characteristics:

    One question I have for you is how are you driving XRS? 

    Best Regards,


  • Thank you for the reply.

    Please check '8.8 Internal Pullup Configuration Requirements' in Technical Reference Manual regarding my enquiry.

    Is it right to understand that GPIO Pullup is enabled if I am using 100Pin Package?
    (Please check the attachment for reference)

    I have also attached the /XRS Input Circuit Diagram as you have requested.

  • Hi Bae,

    Assuming you are using the EPWM pins available for this package (since you are scoping out the EPWM pins), then this TRM note actually does not apply. Within that note,  you can see that it specifies that the statement is applicable to unbonded GPIOs, which is not what you are using. So no need to worry there.

    Can you please provide a scope capture of XRS pin and the EPWM pin together? Would like to see the relationship between the two as well as the activity of XRS signal when driven low.

    Best Regards,