[adrian_b]

The ECC in DDR5/LPDDR5 corrects only internal errors and it has this extra facility only to counteract the degradation of reliability vs. DDR4/LPDDR4, due to smaller cells and faster operation.

It does not really increase much the reliability over older generations, all the mentions about internal ECC are mostly marketing BS.

The ECC that is implemented in the memory controller inside the CPU package protects not only against bit flips in the DRAM arrays, but also against bit errors that happen elsewhere on the long way between memory chips and CPU chips, due to electrical noise, bad seating of CPUs or memory modules in their sockets or cheap sockets whose contacts become oxidized in time.

Due to the increased memory throughput, the links between CPU and memory become more and more sensitive to electrical noise at every new generation.

On laptops or small computers where both the CPUs and the memory chips are soldered on the same PCB, or they are stacked in the same package, ECC is somewhat less important, but on any computer with socketed memory modules ECC should have been mandatory.

[fulafel]

Would you have a reference about ECC in LPDDR5, or Apple specific LPDDR5 features, protecting from internal errors? The sources I found said it only has link error correction, eg https://www.synopsys.com/designware-ip/technical-bulletin/ke...

[adrian_b]

Which kinds of ECC are mandatory and which kinds of ECC are optional can be found only in the JEDEC standards, which are expensive.

On the Synopsys site, both at the link provided by you and in other pages, for DDR5 is mentioned only on-die ECC, which protects only against bit flips during storage, while for LPDDR5 is mentioned only link ECC, which protects only against electrical noise on the PCB traces between the LPDDR5 soldered chips and the CPU soldered chip.

It is likely that on-die ECC is considered more important for DDR5, because the computers that use DDR5 modules are expected to have a larger amount of installed memory, which multiplies the frequency of bit errors during storage, while link ECC is considered more important for LPDDR5, because here the data transfer speed is higher, which multiplies the bit errors due to electrical noise on the PCB link.

On-die ECC can be implemented even if the memory controller of the CPU is not aware of it. Each memory manufacturer may choose to implement on-die ECC, or not, depending on the results of their in-house reliability tests for the storage of the bits in their DRAM chips. The memory manufacturers have no need to mention whether they use internally some form of ECC, because that is transparent for the users of the memories.

Link ECC must be supported by the memory controller and included in the standardized memory interface, so I assume that this is restricted to LPDDR5 memories.