confusion regarding the CMD19 and CMD21 in eMMC/SD card spec

Girish K S girish.shivananjappa at
Thu Sep 15 07:58:05 EDT 2011

Dear all,
             The eMMC 4.5 specification mentions that the CMD21 in
class 2 is a SEND_TUNING BLOCK command. where as the SD card
in the given link says CMD19 in class 2 is the SEND_TUNING_BLOCK
Also the implementation in the drivers/mmc/host/sdhci.c uses the CMD19
Can anybody clarify why there is overlapping in this command number.
May be my understand is wrong.

In the MMC spec the CMD19 is BUS_TEST_W in class 0 and CMD21 is
reserved till eMMC 4.41 version.
But in the SD specification (given link) CMD19 is SEND_TUNING_BLOCK
and CMD21 is reserved.

Thanks and regards
Girish K S

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