Please Explain sector Read Behavior - 16gb - class 4 - micro SD

I have SDHC code in an embedded Kinetis K64 micro (ARM M4).

SDHC operates in 4 bit interface mode with SDHC clock at 40mhz.

SDHC clock run all the time.

My application makes 18 sector (512 bytes) reads from the sd card every 25msec - which means thers ia approx 10msec between activity events.

SD read does NOT use multi-sector reads - I use CMD17

The first sector read following this 10msec of no sd card activity takes over 3msec to complete (all that time is waiting for the sd card to pass its data the the SDHC controller).  All other sector reads during that event only takes 400 usec — that’s a factor of 10 between first read and then remaining reads.

What on the SD card is causing the first read following no activity for 10 msec to take so long (3msec)?

Is there a command that can be sent to the sd to address this behavior.

Please see timing below. – NOTE – READ Activity (CMD17) is active LOW on the trace

Thanks in adavnce for any comments.


SD Read.png