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Boot configuration on STM32F4 Discovery and BOOTx pin voltage threshold

Question asked by simo zz on Nov 3, 2017
Latest reply on Nov 3, 2017 by Clive One

Hello,

I am designing a schematic with an STM32F407VGT6 (TRM DM00031020.pdf) with reference to the STM32F4 Discovery MB997B PCB version for ensuring important details.

 

Checking the Boot configuration and debugging the Discovery board, a few doubts come in mind.

The TRM DM00031020 at page 69 section 2.4 Boot Configuration states the following:


BOOT1 Pin

BOOT0 Pin

Boot Mode

x

0

Main Flash boot

0

1

System memory

1

1

Embedded SRAM boot


From the startup_stm32xx.s files of the Standard Peripheral Libraries, I understand the booot mode used is the Embedded SRAM.

 

Debugging the STM32F4 Discovery Board (schematics UM1472.pdf) bot BOOT0 and BOOT1/PB2 pins are used and are at the same voltage (this justifies the SRAM boot configuration) where VPB2 = VBOOT0 = 143.5 mV (I suppose due to pin output impedance).

 

Looking at STM32F4xx DS (DM00037051.pdf) at table 48 page 114 the VIH threshold is 0.17 VDD + 0.7 so for VDD = 3V, VIH = 1.21.

 

Measuring the resistors R31 and R34 with an ohmmeter both result 3.52 kΩ (on STM32F4 Discovery DS, R34 is marked as 10 k), while R32 and R30 both are 510 Ω (as DS schematics), and for the voltage divider rule when board powers up, both BOOT0 and BOO1 should be at approx. 380 mV.

 

This 381 mV value is neither above the VIH threshold read from DS, neither consistent with the measured value on VPB2 and VBOOT0 (that's why I suppose that the pin output impedance is influencing the measurements).

Could you clarify these mismatches between the measured values and the DS thresholds ?

 

I suspect I am misunderstanding some data but I have to be sure to uderstand the correct configuration.

 

Thank you in advance,

Regards,

Simon

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