TMS320F28335: XRS pin RC requirement

Hi Huihuang,

Our Subject Matter Expert is out of office. Please expect the response by Thursday. Thank you for your patience.

Regards
Srikanth

Hello, I'm pretty sure the same values will apply, but feel free to wait for a more definitive answer from the SME.  That said, based on the schematic of TMDSCNCD28335, it can anecdotally be observed that 2.2kΩ with 100nF should be sufficient:

Incidentally, is this a new project?  If so may I ask what drew the customer to TM320F28335? It is a great device and we have no plans for its end-of-life, but we do have several newer devices which have more robust device training, ease-of-use development tools such as SysConfig, and may even lower the system cost such as TM320F28075.

Hi Joe,

It is MP project. customer want to make sure their use case without any risk .

Customer external RC is :R  is 5.1K, C is 1uF.

I'm very curious , based on below Watchdog block diagram ,the WDRSTn is AND with XRSn  to output Core-reset, why the RC still have this kinds of requirements?

Again, we will allow the SME to confirm when they return but the pull-up resister and capacitor to ground offer protection against noise which could spuriously reset the device.  If the resister is too strong the F28335 device may not be able to adequately administer a reset, but if too weak, the XRSn pin may be more susceptible to noise causing unintended resets.  The value of the capacitor will determine the RC time content which could impact the internal watchdog reset ability of holding the device in reset the required amount of time for a clean device reset.

Huihuang Chen said:

It is MP project. customer want to make sure their use case without any risk .

OK thanks for letting me know.  As mentioned, F28335 is an excellent choice!

Haressh,

Could you help on below question?

1.The Watchdog is same between F28035 and F28335? which is the  XRS external RC requirement for F28335？

2. Customer use case has risk or not?  F28335 ,external RC is :R  is 5.1K, C is 1uF.

3.I understanding the RC to dfferentiate WD-iniated-reset from other forms of reset,but except the WDFLG , any risk for the WDRST failed in customer use case .:F28335 ,external RC is :R  is 5.1K, C is 1uF.

4.from above Watchdog block diagram , it is seems the RC value has no effect for the Core-reset ,may i know my understanding is correct or not?

Haressh,

One more question:

If the Watchdog is same between F28035 and F28335 , do we need to add XRS external RC requirement  in F28335 datasheet like : 2.2 kΩ to 10 kΩ , should be 100 nF or smaller

Hi Huihuang, unfortunately Hareesh had to take a few more days off.  The thread Hareesh provided does suggest the WD are the same between F28035 and F28335.  The main purpose of the resister/capacitor guidance is to distinguish between different reset types which might be more critical for F28035 which has an internal BOR reset.  If the customer doesn't care to distinguish between resets, I don't think a weaker pull-up will be a problem.  I will try to get more specific answers tomorrow.  Can you explain how the customer drives XRSn in their system?  Will they have a supervisor circuit?

Hi Joe,

Yes, customer have external supervisor, so the main concern will be WDRST have issue or not  if F28335 ,external RC is :R  is 5.1K, C is 1uF.

Customer no need to distinguish between different reset types.

Hi Huihuang, I'm fairly certain those values will not be an issue for the WD, but at this point think it best to wait for Hareesh to get back and reply to your questions 1-4 above.  I apologize for the inconvenience and appreciate your patience.