Add info about building for those wanting to use something other than Eclipse to build the firmware. Mention SWO issues with changing clock. Adjust formatting a bit.

README.md

@@ -4,6 +4,16 @@ This is the firmware for the TNC3 version 2.1.1 hardware.
 
 Use Eclipse with CDT and the GNU MCU Eclipse plugins.
 
+If you are porting this to another build platform, you will need to
+build the firmware using the same compiler and linker options.  As
+with most firmware projects, there is a linker script with defines
+the memory layout for for the Flash and SRAM.
+
+Below are example compilation and linking lines for reference:
+
+    arm-none-eabi-gcc -mcpu=cortex-m4 -mthumb -mfloat-abi=softfp -mfpu=fpv4-sp-d16 -O2 -fmessage-length=0 -fsigned-char -ffunction-sections -fdata-sections -fno-inline-functions -fsingle-precision-constant -fstack-usage -fstrict-aliasing -ffast-math -Wall -Wextra -Wlogical-op -Wfloat-equal -g3 -D__FPU_PRESENT=1 -DUSE_HAL_DRIVER -DARM_MATH_CM4 -D__weak=__attribute__((weak)) -DSTM32L433xx -I/usr/arm-none-eabi/include -I../Inc -I../Drivers/STM32L4xx_HAL_Driver/Inc -I../Drivers/CMSIS/Include -I../Drivers/CMSIS/Device/ST/STM32L4xx/Include -I../Middlewares/Third_Party/FreeRTOS/Source/include -I../Middlewares/Third_Party/FreeRTOS/Source/CMSIS_RTOS -I../Middlewares/Third_Party/FreeRTOS/Source/portable/GCC/ARM_CM4F -I../TNC -I../Middlewares/ST/STM32_USB_Device_Library/Class/CDC/Inc/ -I../Middlewares/ST/STM32_USB_Device_Library/Core/Inc -std=gnu11 -Wmissing-prototypes -Wstrict-prototypes -Wbad-function-cast -c -o Src/stm32l4xx_it.o ../Src/stm32l4xx_it.c 
+    arm-none-eabi-g++ -mcpu=cortex-m4 -mthumb -mfloat-abi=softfp -mfpu=fpv4-sp-d16 -O2 -fmessage-length=0 -fsigned-char -ffunction-sections -fdata-sections -fno-inline-functions -fsingle-precision-constant -fstack-usage -fstrict-aliasing -ffast-math -Wall -Wextra -Wlogical-op -Wfloat-equal -g3 -T /home/rob/TNC3/firmware/STM32L433CCUx_FLASH.ld -Xlinker --gc-sections -Wl,-Map,firmware.map --specs=nano.specs -o firmware.elf Drivers/STM32L4xx_HAL_Driver/Src/stm32l4xx_hal.o Drivers/STM32L4xx_HAL_Driver/Src/stm32l4xx_hal_adc.o Drivers/STM32L4xx_HAL_Driver/Src/stm32l4xx_hal_adc_ex.o Drivers/STM32L4xx_HAL_Driver/Src/stm32l4xx_hal_cortex.o Drivers/STM32L4xx_HAL_Driver/Src/stm32l4xx_hal_crc.o Drivers/STM32L4xx_HAL_Driver/Src/stm32l4xx_hal_crc_ex.o Drivers/STM32L4xx_HAL_Driver/Src/stm32l4xx_hal_dac.o Drivers/STM32L4xx_HAL_Driver/Src/stm32l4xx_hal_dac_ex.o Drivers/STM32L4xx_HAL_Driver/Src/stm32l4xx_hal_dma.o Drivers/STM32L4xx_HAL_Driver/Src/stm32l4xx_hal_dma_ex.o Drivers/STM32L4xx_HAL_Driver/Src/stm32l4xx_hal_flash.o Drivers/STM32L4xx_HAL_Driver/Src/stm32l4xx_hal_flash_ex.o Drivers/STM32L4xx_HAL_Driver/Src/stm32l4xx_hal_flash_ramfunc.o Drivers/STM32L4xx_HAL_Driver/Src/stm32l4xx_hal_gpio.o Drivers/STM32L4xx_HAL_Driver/Src/stm32l4xx_hal_i2c.o Drivers/STM32L4xx_HAL_Driver/Src/stm32l4xx_hal_i2c_ex.o Drivers/STM32L4xx_HAL_Driver/Src/stm32l4xx_hal_iwdg.o Drivers/STM32L4xx_HAL_Driver/Src/stm32l4xx_hal_opamp.o Drivers/STM32L4xx_HAL_Driver/Src/stm32l4xx_hal_opamp_ex.o Drivers/STM32L4xx_HAL_Driver/Src/stm32l4xx_hal_pcd.o Drivers/STM32L4xx_HAL_Driver/Src/stm32l4xx_hal_pcd_ex.o Drivers/STM32L4xx_HAL_Driver/Src/stm32l4xx_hal_pwr.o Drivers/STM32L4xx_HAL_Driver/Src/stm32l4xx_hal_pwr_ex.o Drivers/STM32L4xx_HAL_Driver/Src/stm32l4xx_hal_rcc.o Drivers/STM32L4xx_HAL_Driver/Src/stm32l4xx_hal_rcc_ex.o Drivers/STM32L4xx_HAL_Driver/Src/stm32l4xx_hal_rng.o Drivers/STM32L4xx_HAL_Driver/Src/stm32l4xx_hal_rtc.o Drivers/STM32L4xx_HAL_Driver/Src/stm32l4xx_hal_rtc_ex.o Drivers/STM32L4xx_HAL_Driver/Src/stm32l4xx_hal_tim.o Drivers/STM32L4xx_HAL_Driver/Src/stm32l4xx_hal_tim_ex.o Drivers/STM32L4xx_HAL_Driver/Src/stm32l4xx_hal_uart.o Drivers/STM32L4xx_HAL_Driver/Src/stm32l4xx_hal_uart_ex.o Drivers/STM32L4xx_HAL_Driver/Src/stm32l4xx_ll_usb.o Middlewares/ST/STM32_USB_Device_Library/Class/CDC/Src/usbd_cdc.o Middlewares/ST/STM32_USB_Device_Library/Core/Src/usbd_core.o Middlewares/ST/STM32_USB_Device_Library/Core/Src/usbd_ctlreq.o Middlewares/ST/STM32_USB_Device_Library/Core/Src/usbd_ioreq.o Middlewares/Third_Party/FreeRTOS/Source/CMSIS_RTOS/cmsis_os.o Middlewares/Third_Party/FreeRTOS/Source/croutine.o Middlewares/Third_Party/FreeRTOS/Source/event_groups.o Middlewares/Third_Party/FreeRTOS/Source/list.o Middlewares/Third_Party/FreeRTOS/Source/portable/GCC/ARM_CM4F/port.o Middlewares/Third_Party/FreeRTOS/Source/portable/MemMang/heap_3.o Middlewares/Third_Party/FreeRTOS/Source/queue.o Middlewares/Third_Party/FreeRTOS/Source/stream_buffer.o Middlewares/Third_Party/FreeRTOS/Source/tasks.o Middlewares/Third_Party/FreeRTOS/Source/timers.o Src/arm_fir_f32.o Src/arm_fir_fast_q15.o Src/arm_fir_init_f32.o Src/arm_fir_init_q15.o Src/arm_fir_interpolate_init_q15.o Src/arm_fir_interpolate_q15.o Src/arm_offset_q15.o Src/arm_q15_to_float.o Src/freertos.o Src/main.o Src/stm32l4xx_hal_msp.o Src/stm32l4xx_hal_timebase_TIM.o Src/stm32l4xx_it.o Src/system_stm32l4xx.o Src/usb_device.o Src/usbd_cdc_if.o Src/usbd_conf.o Src/usbd_desc.o TNC/AFSKModulator.o TNC/AFSKTestTone.o TNC/Afsk1200Demodulator.o TNC/AfskDemodulator.o TNC/AudioInput.o TNC/AudioLevel.o TNC/DCD.o TNC/Demodulator.o TNC/Digipeater.o TNC/FilterCoefficients.o TNC/Fsk9600Demodulator.o TNC/Fsk9600Modulator.o TNC/Goertzel.o TNC/Golay24.o TNC/HdlcDecoder.o TNC/HdlcFrame.o TNC/IOEventTask.o TNC/Kiss.o TNC/KissHardware.o TNC/KissTask.o TNC/LEDIndicator.o TNC/Log.o TNC/M17.o TNC/M17Demodulator.o TNC/M17Encoder.o TNC/M17Modulator.o TNC/ModulatorTask.o TNC/NullPort.o TNC/PortInterface.o TNC/PowerMode.o TNC/SerialPort.o TNC/UsbPort.o TNC/bm78.o TNC/bm78_eeprom.o newlib/_exit.o newlib/_sbrk.o newlib/_syscalls.o startup/startup_stm32l433xx.o 
+
 # Debugging
 
 Logging is enabled in debug builds and is output via ITM (SWO).  The
@@ -18,6 +28,10 @@ To read from this pipe, open a terminal and run:
 If you change the MCU's core clock, you need to adjust the timing in the
 `stlink-tnc3.cfg` config file.
 
+The TNC3 runs at 48MHz on startup.  It may switch to 80MHz for modulation types
+such as 9600 or M17 which require more speed.  This makes using SWO challenging
+as it cannot handle changes in core speeds.
+
 # Installing firmware
 
 Firmware can be installed via the on-board ST/LINK port or via USB DFU.
@@ -30,10 +44,11 @@ Firmware can be installed via the on-board ST/LINK port or via USB DFU.
 
  2. Download the ELF file from the release (or that you have built from source).
 
- 3. Plug the TNC into a USB port and turn the TNC on.  You should see a USB serial port enumerated.
+ 3. Plug the TNC into a USB port and turn the TNC on.  You should see a USB
+    serial port enumerated.
 
- 4. Put the TNC into DFU mode by pressing the DFU button on the side.  The TNC
-will only enter DFU mode when plugged into a USB port.
+ 4. Put the TNC into DFU mode by pressing the DFU/boot button on the side.  The
+    TNC will only enter DFU mode when plugged into a USB port.
 
  ![TNC3 Diagram](https://s3.amazonaws.com/mobilinkd/TNC3/TNC3_Diagram.png)
 
@@ -41,9 +56,10 @@ will only enter DFU mode when plugged into a USB port.
 
  5. You should see the serial port device go away and a new DFU device appear.
 
- 6. Run the STM32CubeProgrammer from the command-line. (Replace "firmware.elf" with the appropriate firmware filename.)
+ 6. Run the STM32CubeProgrammer from the command-line. (Replace "firmware.elf"
+    with the appropriate firmware filename. It must be in ELF format.)
 
     ./STM32_Programmer_CLI -c port=USB1 -d firmware.elf -v -g 0x8000000
 
  7. When that is complete, the DFU device will disappear and the serial port
-device will re-appear.
+    device will re-appear.