STM32L432Kx Flash Size

The STM32L432Kx is available with two different flash sizes. The STM32L432KB has got 128 kB while the STM32L432KC has got 256 kB. I recently created a USB "key" using the STM32L432. The reason for using this specific STM32 MCU was twofold:

1. It is small
2. It's got a built-in 48 MHz oscillator precise enough to use USB without an external crystal

Unfortunately, while I wanted the 256 kB version, only the 128 kB version was available.

The STM32 Blue Pill boards mostly use Chinese clone MCUs but there has been a lot of writing about the original ones having 128 kB of flash rather than the 64 kB specified by the datasheet. There is even a test tool available which can test for the hidden flash.

Inspired by this, I decided to test the STM32L432KB on my own board. As per the datasheet this processor is supposed to have 128 kB of flash ranging from 0x8000000 to 0x801FFFF.

Testing

Checking device

lth@ncpws04:~/tmp$ st-info --probe
Found 1 stlink programmers
  version:    V2J45S31
  serial:     066CFF3933584B3043184346
  flash:      131072 (pagesize: 2048)
  sram:       49152
  chipid:     0x435
  dev-type:   STM32L41x_L42x

Generating Test Data

First step is to create a 256 kB file with random data.

lth@ncpws04:~/tmp$ dd if=/dev/urandom of=random.bin bs=1k count=256
256+0 records in
256+0 records out
262144 bytes (262 kB, 256 KiB) copied, 0.0012241 s, 214 MB/s

Writing data to MCU

The random.bin file is of a size which can be flashed on both the STM32L432KB and STM32L432KC.

First step is to flash normal:

lth@ncpws04:~/tmp$ st-flash write random.bin 0x8000000
st-flash 1.8.0
2024-09-02T10:30:01 INFO common.c: STM32L41x_L42x: 48 KiB SRAM, 128 KiB flash in at least 2 KiB pages.
file random.bin md5 checksum: 5f7ebd5c822e7408e6279445eb1444a, stlink checksum: 0x01fe53dd
2024-09-02T10:30:01 INFO common_flash.c: Attempting to write 262144 (0x40000) bytes to stm32 address: 134217728 (0x8000000)
2024-09-02T10:30:01 ERROR common_flash.c: The size exceeds the size of the flash (0x00020000 bytes available)
stlink_fwrite_flash() == -1

As expected this will fail. The MCU reports 128 kB of flash and we attempt to write 256 kB. We can however instruct st-flash to ignore that and attempt to write the entire file:

lth@ncpws04:~/tmp$ st-flash --flash=256k write random.bin 0x8000000
st-flash 1.8.0
2024-09-02T10:30:26 INFO common.c: STM32L41x_L42x: 48 KiB SRAM, 128 KiB flash in at least 2 KiB pages.
Forcing flash size: --flash=0x00040000
file random.bin md5 checksum: 5f7ebd5c822e7408e6279445eb1444a, stlink checksum: 0x01fe53dd
2024-09-02T10:30:26 INFO common_flash.c: Attempting to write 262144 (0x40000) bytes to stm32 address: 134217728 (0x8000000)
EraseFlash - Page:0x10 Size:0x800 -> Flash page at 0x8008000 erased (size: 0x800EraseFlash - Page:0x11 Size:0x800 -> Flash page at 0x8008800 erased (size: 0x800EraseFlash - Page:0x12 Size:0x800 -> Flash page at 0x8009000 erased (size: 0x800EraseFlash - Page:0x13 Size:0x800 -> Flash page at 0x8009800 erased (size: 0x800EraseFlash - Page:0x14 Size:0x800 -> Flash page at 0x800a000 erased (size: 0x800EraseFlash - Page:0x15 Size:0x800 -> Flash page at 0x800a800 erased (size: 0x800EraseFlash - Page:0x16 Size:0x800 -> Flash page at 0x800b000 erased (size: 0x800EraseFlash - Page:0x17 Size:0x800 -> Flash page at 0x800b800 erased (size: 0x800EraseFlash - Page:0x18 Size:0x800 -> Flash page at 0x800c000 erased (size: 0x800EraseFlash - Page:0x19 Size:0x800 -> Flash page at 0x800c800 erased (size: 0x800EraseFlash - Page:0x1a Size:0x800 -> Flash page at 0x800d000 erased (size: 0x800EraseFlash - Page:0x1b Size:0x800 -> Flash page at 0x800d800 erased (size: 0x800EraseFlash - Page:0x1c Size:0x800 -> Flash page at 0x800e000 erased (size: 0x800EraseFlash - 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2024-09-02T10:30:29 INFO flash_loader.c: Starting Flash write for F2/F4/F7/L4
2024-09-02T10:30:29 INFO flash_loader.c: Successfully loaded flash loader in sram
2024-09-02T10:30:29 INFO flash_loader.c: Clear DFSR
2024-09-02T10:30:35 INFO common_flash.c: Starting verification of write complete
2024-09-02T10:30:37 INFO common_flash.c: Flash written and verified! jolly good!

That went well - no errors reported. Now let's try to read that flash into a file:

Reading data from the MCU

To be absolutely certain, read the data back into a new file:

lth@ncpws04:~/tmp$ st-flash --flash=256k read random2.bin 0x8000000 0x40000
st-flash 1.8.0
2024-09-02T10:31:51 INFO common.c: STM32L41x_L42x: 48 KiB SRAM, 128 KiB flash in at least 2 KiB pages.
Forcing flash size: --flash=0x00040000
2024-09-02T10:31:51 INFO common.c: read from address 0x08000000 size 262144

No complaints!

Compare data

Finally we can verify that the data read from the MCU matches the data we wrote to the MCU:

lth@ncpws04:~/tmp$ md5sum random*
5f7ebd5c822e07408e6279445eb1444a  random2.bin
5f7ebd5c822e07408e6279445eb1444a  random.bin

They are identical, so it is absolutely confirmed that this particular STM32L432KB is in fact a STM32L432KC which identifies as a STM32L432KB.

Miscellaneous Links

• STM32L432 Datasheet
• Bluepill Diagnostics Tool