I've connected second SD card via SDIO 1-bit bus. Works well with sdio overlay. But, how I can change clock frequency for this interface? I want to make some diagnosis with my oscilloscope and need a clock lowest as possible.
I've tried change the parameter sdio_overclock, but nothing was changed. Clock frequency is always 25MHz.
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pi@raspberrypi:~ $ sudo cat /sys/kernel/debug/mmc1/ios clock: 25000000 Hz actual clock: 25000000 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: 0 (1 bits) timing spec: 0 (legacy) signal voltage: 0 (3.30 V) driver type: 0 (driver type B)