Microcontroller

Microcontroller selection

The microcontrollers compatible with the most recent RAMN firmware are:

  • STM32L562CET6 (512kB flash, with hardware cryptography).

  • STM32L552CET6 (512kB flash, no hardware cryptography).

  • STM32L552CCT6 (256kB flash, no hardware cryptography).

Because STM32L552CCT6 only has 256kB flash, it does not support UDS reprogramming, but it supports other reprogramming interfaces. The same firmware is used for all the microcontrollers above; RAMN automatically detects how much flash is available, and does not use hardware cryptography.

RAMN’s firmware does not support STM32L4 microcontrollers anymore, but the PCB is still compatible with them. Old source code for STM32L4 family, used in the BETA version of RAMN, is available on request but is not maintained and has very limited features.

RAMN is not “officially” compatible with STM32U5 microcontrollers. If you want to use STM32U5 microcontrollers, the only change needed is to update the PCB to add capacitors to the PB11 pins (which became “VCAP” in STM32U5 series). The source code is mostly compatible, but many of the code generation features used with STM32L5 are unavailable with STM32U5, so some porting work is needed. Binary files are not compatible (you cannot flash an STM32L5 firmware to a STM32U5 device). Feel free to contact us if you need guidance on how to use STM32U5 microcontrollers.

Note

For STM32L microcontrollers, the last four letters of the part number are used to specify:

  • Pin count (C -> 48 pins)

  • Flash Memory size (C -> 256 KB, E -> 512 KB)

  • Package ( T -> LQFP)

  • Temperature range (3 -> from -40 to 125 degrees C, 6 -> from -40 to 85 degrees C)

RAMN’s PCB requires the microcontrollers to be in a LQFP48 package.

Surrounding circuitry

../_images/microcontroller.png

Circuitry surrounding the ECUs’ microcontroller.

As much as possible, we try to follow the hardware design guidelines of STMicroelectronics. The microcontroller VDD pins are connected to individual 100nF decoupling capacitors, and a common 10uF capacitor. The RST pin is also connected to a 100nF capacitor. To save space, the VDDA pin is NOT connected to external decoupling capacitors and ferrite beads. Similarly, VBAT does not feature a decoupling capacitor.

Clock

A crystal from the NX3225GD is selected as the clock source of the microcontroller. The NX3225GD family is automotive-grade, is present on the STM32 compatible crystals list and has also been selected on some Nucleo designs . NX3225GD crystals are widely available in 8MHz and 10MHz version. RAMN can be used with both, but because of the internal PLL circuitry, the 8MHz does not allow reaching the maximum system clock of 110MHz for the STM32L5 family. Therefore, the 10MHz crystal is preferred.

The NX3225GD crystal has a load capacitance of 8pF, which is relatively low compared to similar products. Assuming a stray capacitance of 3pF, two external 10pF capacitors were selected to approximate a 8pF load (3 + 10/2 = 8).

Although not ideal, the layout follows best practice by keeping the crystal close to the microcontrollers, and shielding the clock lines from other signals.

../_images/ecu_clock.png

Clock circuitry on every ECU.

Boot Selection Resistors

There are boot selection resistors, because RAMN used to be compatible with both STM32L4 and STM32L5, which have different boot sequences.

On STM32L4 and STM32L5 families, the BOOT0 pin is used by default to specify in which mode the microcontroller should start. Resistors R8 and R9 can be used to select the BOOT0 pin state of ECU A. Only ONE of those resistors should be populated at a time.

../_images/ecu_boot.png

Boot mode selection resistors of ECU A.

The boot patterns are different for STM32L4 (Pattern 6) and STM32L5 (Pattern 12)

STM32L4 will start the embedded bootloader when the flash memory is unprogrammed. This means a freshly-made board can directly be programmed over USB, no matter the state of BOOT0. Therefore, we use R9 to pull the BOOT0 pin down, and R8 is not populated. On the first connection, ECU A will start in DFU mode and be ready for programming over USB. Once ECU A is programmed, and because BOOT0 is pulled down, the board will launch the programmed firmware from the next reboot.

STM32L5 will NOT start the embedded bootloader even if the flash memory is unprogrammed. To ensure a fresh board can be programmed over USB, we therefore need to use R8 to pull the BOOT0 pin up and force the ECU to start in DFU bootloader. R9 is left unpopulated. This means that ECU A will always start in STM32 bootloader mode, until the option bytes are overwritten with new values forcing another boot process.

The BOOT0 pin of ECU A can be accessed externally through a test pad next to the USB port. However, the BOOT0 pin is ignored by ECU A once a valid firmware has been flashed.

../_images/boot0_testpad.png

ECU A Boot0 test pad.

The BOOT0 pin of ECUs B, C, and D are controlled by ECU A. ECU A should ensure the BOOT0 pin of each ECU is properly set before activating their power supply.