STM32N657 IO 3.3V tolerant?

[matt-crc]

Hello,

I'm using the STM32N657 processor and have the HEXASPI device running at 1.8V.  

VDDIO2 is connected to 1V8 supply (which affects: PO[5..0] and PP[15..0]

The FMC is placing the A23 line on pin PO1 (this is the only option for the FMC-A23) and the FMC device is running at 3.3V.   Can I place a pullup to 3.3V on PO1, or will this affect/blow  the GPIO connected to 1.8V -OR- do I need to place a voltage translator on that pin?

[AScha.3]

Hi,

You cannot pull up a fast output line to another level, the internal protection diodes would prevent it. And you probably want a fast signal here, nothing slow with pullups. So take maybe a single gate as a level shifter for getting a fast output signal.

[RomainR. · Accepted solution]

Hi @matt-crc 

As explained in DS14791 Rev7:

Absolute Maximum ratings (value never to be reached) Table 21. Voltage characteristics. On a TT GPIO in 1.8V HSLV mode (corresponding to programmed OTP124 HCONF and VDDIOxVRSEL = 1) the max VIN voltage = 1.98V

Table 24. General operating conditions, VIN when I/O TT_xx in the 1.8V range, max = min VDD + 0.3V.

You do not indicate the package you are using. On the VFBGA264, you have the FMC_A23 option on PB3 or PD1.

Best regards,

Romain

[matt-crc]

Hi @RomainR. 

1- yes it is the VBGA264    STM32N657X0H3Q

2- FMC-A23 is not available on A14-PD1, F13-PB3, or T17-PN2.  I attached the .ioc file if you wanted to see it.

3- I want to run the FMC at 200MHz with the address setup time = 1 cycle.  I'm not sure if a voltage converter will skew address decoding (if that is the only address line going to a voltage translator).

At this point, I'm debating if i should just remove A23 all together from the design... sigh

4- Can you confirm that on the N657, the FMC can run at 200MHz?  All I found in the reference manual was a transfer rate of 800Mbytes/sec on the FMC.

thanks, Matt

[LCE]

Forget any fiddling at these speeds with pull-ups, and - what you said - even level converters might have a too high propagation delay, unless all signals are going through similar devices - which would probably take too much space.

So better change the MCU package - or as you said, check if you can remove A23. 

[RomainR.]

Hi @matt-crc 

You have assigned PB3 and PD1 for other FMC signals in your .ioc (FMC_A7 and MC_FBL1)
This is why you cannot use them for FMC_A23.
But it is available in AF14 as explained in Table 20. Alternate functions: AF8 to AF15

The FMC timing can be found in DS14791 Rev7 in 5.3.19 FMC characteristics where all timing
are based on Tfmc_ker_ck = fmc_ker_ck

If you refer to RM0486 in Table 73. Peripheral clock distribution summary (continued) indicates
max kernel clock FMC = 200MHz

You already use the biggest package. You must choose between XSPI or FMC, both seem not a possible use case.

Best regards,

Romain,