Hi Alex, Thank you for your support! I checked power supply and these pins, but they seemed to be fine. Then I found a high correlation between the signal on CS pin and START pin (they are physically close to each other). When I resumed data retrieval, START pin is pulled high and low exactly like CS pin does, but random spikes still present and sometimes START pin stayed low for a while (which should be the normal behavior). I assembled and tested another board and did not encounter this problem. Maybe the chip or the board is damaged? For a temporary fix I pulled START pin high with a pullup resistor and it's now working fine. START pin stayed high and is not driven low.
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