• State Resolved
  • Locked Locked
  • Replies 2 replies
  • Subscribers 82 subscribers
  • Views 394 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

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.

F28M35H52C: unable to assert a device reset by an open-drain device connected to XRS (pin 4) and ARS (pin 144)

Lorenzo Rebonato
Prodigy 20 points
Part Number: F28M35H52C


Hello everybody,

the Concerto Microcontroller F28M35H52C has to external reset pins: XRS and ARS (both active low).

We externally tied together these two pins with a board trace, as recommended by TI (see SPRS742J, paragraph 6.8).

TI also clarifies that these pins can receive an external reset signal, provided that the pins are driven by an open-drain device,

with a pull-up resistor between 2.2 Kohm and 10Kohm placed between XRS and VDDIO (see SPRS742J, Table 4-1).

This is exactly what we did (2.5Kohm pull-up resistor and 100nF capacitor), but whenever the reset input is forced low by the external driver, the signal level changes from 3.3V (VDDIO) to 2.5V and the device does not reset.

Basically, it seems that the XRS and ARS pins (or one of them) behave like a push-pull output, instead of an I/OD (Input/Open-Drain) as stated in the document SPRS742J, Table 4-1.

Any suggestions about this strange behaviour?

Is it possible that these two pins can be SW-configured as outputs, despite what the TI documentation claims (SPRS742J par. 6.16)?

Thanks and Best Regards,

LR

  • 0
    TI__Guru* 95315 points
    Do you have a series resistor between the external driver and the -XRS pin? Can you provide the schematics of all the components connected to the -XRS pin?
  • 0 in reply to Hareesh Janakiraman
    Prodigy 20 points
    Hello Hareesh,
    thanks for your feedback.
    Actually the problem was due to a conflict on the reset signal: the reset input of the F28M35 is controlled by wired-OR Open-Drain supervisors, but the same reset signal was also driven by a push-pull output with a Schottky diode in between. The OD driver was pulling the line down, while the upper MOSFET of the push-pull output was pulling up (as a result, reset signal level at about 2.5V...). Thanks.