I connected Flash externally and observed the SPI signal. This is still my imagination.
The roles of the three functions needed to send commands are
flashTransport.begin();
CS–>HIGH, CS–>LOW, OUT 0x05, IN StatusRegister, CS–>HIGH
flashTransport.runCommand();
CS–>LOW, OUT 0xAB, CS–>HIGH
flashTransport.end(); flashTransport.
CS–>OPEN
Flash only accepts command 0xAB if it is in Deep PowerDown mode.
See datasheet 10.30 Deep Power-down (DP)
“Once the DP instruction is set, all instructions will be ignored except the Release from Deep Power-down mode (RDP) and Read Electronic Signature (RES) instruction.”
The Adafruit library does not support either command 0xB9 or 0xAB. Flash may receive 0x05 and hang up.
SPI_2.transfer(0xAB); is working fine.
I have not yet gotten around to the tips and research you gave me.