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Question asked by Valentina Fernandez Alanis on Jul 22, 2015
Latest reply on Jul 28, 2015 by Bio_TICFSL



We have a custom board based on IMX6 Sabre Auto. The board supports voltage switching from 3.3 V to 1.8 V using the VSELECT signal in the SD card interfaces. However, when inserting a Lexar 633x 128 GB microSD card and using the hdparm utility the output shows the following ( speed is too slow considering that SDR104 max bus speed is 104 MB/s) :




Timing cached reads:   656 MB in  2.00 seconds = 327.47 MB/sec

Timing buffered disk reads: 118 MB in  3.01 seconds =  39.18 MB/sec


We can confirm that the voltage is switching to 1.8V (measuring voltage) and that linux is identifying the microSD card as UHS SDR104 with the following test:


root@localhost:~# cat /sys/kernel/debug/mmc2/ios 


clock:          198000000 Hz


actual clock:   198000000 Hz

vdd:            21 (3.3 ~ 3.4 V)

bus mode:       2 (push-pull)

chip select:    0 (don't care)

power mode:     2 (on)

bus width:      2 (4 bits)

timing spec:    6 (sd uhs SDR104)

signal voltage: 0 (1.80 V)




Why is the SD access speed so slow? We are using kernel 3.14.28