How to restore STM32H7 CRC registers to continue CRC calculation

[Propwash]

Is there any doco or app note on how to save/restore a STM32H7 CRC calculation (e.g. breaking up long CRC calculation into separate segments that requires re-initialization of CRC HW between segments)?

[Saket_Om]

Hello ​@Propwash 

To continue a CRC calculation on STM32H7 across segments or after re‑initializing the CRC hardware, you can:

Before re-initialization / break:

• Read the current CRC result from the CRC_DR register.

• This value represents the intermediate CRC for the data processed so far.

After re-initialization of the CRC peripheral:

• Write the saved intermediate CRC value into the CRC_INIT register.

• This sets the starting (initial) CRC state for the next segment, effectively resuming the calculation where you left off.

Process the next data segment:

• Feed the next chunk of data into the CRC peripheral as usual.

• The final CRC result after all segments are processed will be in CRC_DR.

This save/restore via CRC_DR → CRC_INIT allows you to break a long CRC calculation into segments or survive a CRC peripheral reconfiguration while preserving continuity of the CRC computation.

[Propwash]

@Saket_Om

After I reset the CRC HW, I tried to set CRC_INIT to the value of the previous CRC_DR and continue but that did not produce the results I expected.

I created a utility in Nuttx running on Nucleo-H753ZI that reads/write the CRC registers and what follows is that while CRC’ng data consisting of 0x61626364 0x65666768, then resetting the CRC HW, setting initial value to the CRC_DR register value from CRC_DR of just 0x61626364, writing 0x65666768 to CRC_DR (to complete the CRC) and reading back CRC_DR, expecting result to match match (but it doesn’t):

NuttShell (NSH) NuttX-13.0.0-RC0
nsh> uname -a
NuttX 13.0.0-RC0 76838fa1b2 Jul  2 2026 13:41:40 arm nucleo-h753zi
nsh> pokecrc32 --help                                                           
pokecrc32: usage:                                                               
  -d    Turn on debug                                           
  -f     Force addresses to only be register names                              
  -b    Read/Write a byte                                       
  -h    Read/Write a short                                      
  -l     Read/Write a long                                                                    
  -r <addr>              Read at physical address <addr>                         
  -w <addr> <val>  Write <val> at physical address <addr>                  
Note: <addr> and <val> must be hex(w/o '0x' prefix)                              
      when reading a registers, '~' before register                             
      indicates to flip the bits. On write '~' before                           
      value indicates to flip its bits

                                        

Verify CRC clock is enabled (verify CRCEN is set in RCR_AHB4ENR)

nsh> pokecrc32 -r rcc_ahb4enr
580244e0 /l => 000800ff

Hard reset CRC (set/clear CRCRST in RCC_AHB4RSTR)

nsh> pokecrc32 -w rcc_ahb4rstr 80000 -w rcc_ahb4rstr 0
58024488 /l <= 00080000
58024488 /l <= 00000000

Reset/initialize CRC_CR to 0xe1(and read back to verify RESET clears):

• Bit reverse output data
• Bit reversal input data by word
• 32-bit polynomial
• Set reset bit

nsh> pokecrc32 -w crc_cr a1 -r crc_cr
58024c08 /l <= 000000a1
58024c08 /l => 000000a0

nsh> # Compute CRC of 'abcd' (expect raw 127d32ee; inverted ed82cd11)
nsh> pokecrc32 -w crc_dr 61626364 -l -r crc_dr -r ~crc_dr
58024c00 /l <= 61626364
58024c00 /l => 127d32ee
58024c00 /l => 127d32ee(~= ed82cd11)

Note /usr/bin/crc32 produces matching ed82cd11 for four byte 'abcd'

nsh> # Append 'efgh' (expect raw 5110d5af; inverted aeef2a50)
nsh> pokecrc32 -w crc_dr 65666768 -l -r crc_dr -r ~crc_dr
58024c00 /l <= 65666768
58024c00 /l => 5110d5af
58024c00 /l => 5110d5af(~= aeef2a50)

Note /usr/bin/crc32 produces matching aeef2a50 for eight byte 'abcdefgh'

Now Reset CRC HW, Intialize CRC_CR, and set CRC_INIT to result from previous
CRC_DR value for 'abcd'

nsh> # Hard Reset CRC
nsh> pokecrc32 -w rcc_ahb4rstr 80000 -w rcc_ahb4rstr 0
58024488 /l <= 00080000
58024488 /l <= 00000000
nsh> # Soft Reset CRC
nsh> pokecrc32 -w crc_cr a1 -r crc_cr
58024c08 /l <= 000000a1
58024c08 /l => 000000a0
nsh> # Set Initial value to CRC result of 'abcd'
nsh> pokecrc32 -w crc_init 127d32ee
58024c10 /l <= 127d32ee

Now feed in 'efgh' which should produce the same CRC_DR value as
previously:

nsh> # CRC of 'efgh' (expect raw 5110d5af; inverted aeef2a50)
nsh> pokecrc32 -w crc_dr 65666768 -l -r crc_dr -r ~crc_dr
58024c00 /l <= 65666768
58024c00 /l => 2abae3f7
58024c00 /l => 2abae3f7(~= d5451c08)

Note the resultant CRC does not match the expected aeef2a50.  Any idea why this doesn’t work as expected? Thanks in advance!