M_CAN message RAM initialization AppNote - was: Re: [PATCH V3 3/3] can: m_can: workaround for transmit data less than 4 bytes

[Dong Aisheng]

On Wed, Nov 05, 2014 at 07:15:10PM +0100, Oliver Hartkopp wrote:
> Hi all,
> 
> just to close this application note relevant point ...
> 
> I got an answer from Florian Hartwich (Mr. CAN) from Bosch regarding
> the bit error detection found by Dong Aisheng.
> 
> The relevant interrupts IR.BEU or IR.BEC monitor the message RAM:
> 
> Bit 21 BEU: Bit Error Uncorrected
> Message RAM bit error detected, uncorrected. Controlled by input
> signal m_can_aeim_berr[1] generated by an optional external parity /
> ECC logic attached to the Message RAM. An uncorrected Message RAM
> bit error sets CCCR.INIT to ‘1’. This is done to avoid transmission
> of corrupted data.
> 
> 0= No bit error detected when reading from Message RAM
> 1= Bit error detected, uncorrected (e.g. parity logic)
> 
> Bit 20 BEC: Bit Error Corrected
> Message RAM bit error detected and corrected. Controlled by input
> signal m_can_aeim_berr[0] generated by an optional external parity /
> ECC logic attached to the Message RAM.
> 
> 0= No bit error detected when reading from Message RAM
> 1= Bit error detected and corrected (e.g. ECC)
> 
> ---
> 
> The Message RAM is usually equipped with a parity or ECC functionality.
> But RAM cells suffer a hardware reset and can therefore hold
> arbitrary content at startup - including parity and/or ECC bits.
> 
> So when you write only the CAN ID and the first four bytes the last
> four bytes remain untouched. Then the M_CAN starts to read in 32bit
> words from the start of the Tx Message element. So it is very likely
> to trigger the message RAM error when reading the uninitialized
> 32bit word from the last four bytes.
> 
> Finally it turns out that an initial writing (with any kind of data)
> to the entire message RAM is mandatory to create valid parity/ECC
> checksums.
> 
> That's it.
> 

Thanks for sharing this information.
Does it mean this issue is related to the nature of Message RAM and is
supposed to exist on all M_CAN IP versions?

> Regards,
> Oliver
> 

Regards
Dong Aisheng