Linux/AM3352: SYSBOOT pins left floating

Mohsen Khayami

Part Number: AM3352

Tool/software: Linux

Hi champs

I know in the TRM it indicates that SYSBOOT pins [11:6] are dont care if we use boot mode 11100b. What i am worried about is that in all the TI platforms we pull all the sysboot pins either high or low no matter what boot mode we use. So i am trying to double check to make sure that those sysboot pins can be left floating.

Also if i use pins U1 and U2 for uart2 CTS and RTS do i have to pull them high or low because they are used as sysboot pins when the processor boots? I know that the sysboot pins are read on the rising edge of the reset but if these are don't care then it should not matter correct?

We are seeing some strange behavior from a customer board and trying to see what can cause the strange behavior.

Thanks

Regards

Mohsen

over 6 years ago

0 Biser Gatchev-XID over 6 years ago

TI__Guru**** 393215 points

Hi,

Not a good idea. All SYSBOOT pins are High-Z at reset release time, and need to be pulled externally.

0 Mohsen Khayami over 6 years ago in reply to Biser Gatchev-XID

TI__Mastermind 22391 points

Thanks Biser

But in the TRM it says don't care for the ones I mentioned so it should be fine. Can you explain why they have to be tied low or high if it is a don't care for my boot case please.

Thanks

0 Biser Gatchev-XID over 6 years ago in reply to Mohsen Khayami

TI__Guru**** 393215 points

Hi Mohsen,

Sorry for the late reply, I have been OoO last week.

In my understanding "don't care" does not implicate that the pins can be left floating, just that they can be tied either high or low, whichever is more convenient. So far I haven't seen a design with SYSBOOT pins left floating.

0 JJD over 6 years ago in reply to Biser Gatchev-XID

TI__Guru* 93381 points

Mohsen, for boot, it shouldn't matter, as whatever is value is latched is a don't care for that boot mode. However, those signals will default to GPIO inputs, so in the bootloader i would ensure the internal pulls are enabled as soon as possible to avoid excessive leakage on those signals.

As for your UART question, any signals used for a particular boot mode must be taken care of on the board. However, UART boot only uses RX and TX, so CTS/RTS should not matter during boot.

Regards,

James

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Linux/AM3352: SYSBOOT pins left floating