I have a BL600 (with v1.5.70.0 firmware loaded onto it) connected to another system via UART as a cable replacement. When the other system is in sleep mode, UART data lines are low. If SIO_22 is held low, it seems to prevent the BL600 from booting. I

Answer

The device should power up even if SIO_22 is held low. Please be aware that you will not be able to talk to it or send it commands since the UART_RX line is being held low. It won’t be responsive because when UART_RX is low for a long time that is detected as an incoming BREAK and the firmware is designed to put the module into deep sleep mode, but it should still boot. If you have an Autorun app that boots with SIO_22 held low and asserts the BREAK line then you should see it in Autorun mode. Otherwise, you will see nothing out of the module even though it is powered.