Hi,
I've looked around the DevZone for answers to this, and haven't quite seen an answer, so I'm going to post my solutions here.
I am using the secure bootloader from Nordic SDK 15.3. I am combining all three transports (BLE, UART, USB) into a single bootloader. The first thing to understand about this is that all transports are initialized when the bootloader starts up. There is no need to "select" one. There are all always active. So, just start DFUing to the transport of your choice.
First the simple one: Combining USB with UART:
This fails in the linker because both transport declare m_payload_pool using NRF_BALLOC_DEF, which does not define its variables using the static keyword. Simply rename one or both of the variables. I used m_payload_pool_uart and m_payload_pool_usb.
Second: Combining USB with BLE:
This one is trickier. The problem is usb_dfu_transport_init() will fail with assertion if the softdevice is already started. Whether the softdevice is started or not depends on whether ble_dfu_transport_init() was called first, which depends on the linker. See this code comment for NRF_SECTION_ITEM_REGISTER (which is used by DFU_TRANSPORT_REGISTER):
* @note The order in which variables are placed in a section is dependent on the order in
* which the linker script encounters the variables during linking.
Hmm, what to do? How you can guarantee execution order? I decided it's better not to try and not register for power events.
I set APP_USBD_CONFIG_POWER_EVENTS_PROCESS to 0 in sdk_config.h. I removed the USBD_POWER_DETECTION #define completely from nrf_dfu_serial_usb.c. I removed the call to nrf_drv_power_init() in usb_dfu_transport_init(). The if (USBD_POWER_DETECTION) I removed so only the else clause remained. And finally after app_usbd_enable() and app_usbd_start() I added the following code, directly in usb_dfu_transport_init():
if (m_observer)
{
m_observer(NRF_DFU_EVT_TRANSPORT_ACTIVATED);
}
