Is it mandatory to have a reset in BT_REG_ON during BT firmware download stage in IF573?
Yes, it's mandatory to do this.
While you design IF573 PCB module, need to have a GPIO to control BT_REG_ON; with M.2 module, wire to W_DISABLE2. During BT firmware download, need to have reset signal to indicate the firmware loading event.
Here is an example of the script to download BT firmware.
# de-assert BT_REG_ON (assert W_DISABLE2# on M.2 interface)
gpioset 5 7=0
# Start firmware download in background
brcm_patchram_plus --no2bytes --autobaud_mode --baudrate 921600 \
--use_baudrate_for_download --enable_hci \
--patchram /lib/firmware/cypress/CYW55560A1.hcd /dev/ttymxc0 &
# assert BT_REG_ON (de-assert W_DISABLE2# on M.2 interface)
gpioset 5 7=1