README
======

This is the README file for the port of NuttX to the Mikroe Clicker2 STM32
board based on the STMicro STM32F407VGT6 MCU.

Reference: https://shop.mikroe.com/development-boards/starter/clicker-2/stm32f4

Contents
========

o Serial Console
o LEDs
o Buttons
o Using JTAG
o Configurations

Serial Console
==============

The are no RS-232 drivers on-board. An RS-232 Click board is available:
https://shop.mikroe.com/click/interface/rs232 or you can cannot an off-
board TTL-to-RS-232 converter as follows:

USART2: mikroBUS1 PD6/RX and PD5/TX
USART3: mikroBUS2 PD9/RX and PD8TX

GND, 3.3V, and 5V. Are also available

By default, USART3 on mikroBUS2 is used as the serial console in each
configuration unless stated otherwise in the description of the
configuration.

LEDs
====

The Mikroe Clicker2 STM32 has two user controllable LEDs:

LD1/PE12, Active high output illuminates
LD2/PE15, Active high output illuminates

If CONFIG_ARCH_LEDS is not defined, then the user can control the LEDs in any
way. If CONFIG_ARCH_LEDs is defined, then NuttX will control the 2 LEDs on
board the Clicker2 for STM32. The following definitions describe how NuttX
controls the LEDs:

SYMBOL Meaning LED state
LD1 LD2
------------------- ----------------------- -------- --------
LED_STARTED NuttX has been started OFF OFF
LED_HEAPALLOCATE Heap has been allocated OFF OFF
LED_IRQSENABLED Interrupts enabled OFF OFF
LED_STACKCREATED Idle stack created ON OFF
LED_INIRQ In an interrupt N/C ON
LED_SIGNAL In a signal handler No change
LED_ASSERTION An assertion failed No change
LED_PANIC The system has crashed OFF Blinking
LED_IDLE STM32 is is sleep mode Not used

Thus is LD1 is illuminated, the Clicker2 has completed boot-up. IF LD2
is glowly softly, then interrupts are being taken; the level of illumination
depends amount of time processing interupts. If LD1 is off and LD2 is
blinking at about 2Hz, then the system has crashed.

Buttons
=======

The Mikroe Clicker2 STM32 has two buttons available to software:

T2/E0, Low sensed when pressed
T3/PA10, Low sensed when pressed

Using JTAG
==========

The Clicker2 comes with the mikroBootloader installed. That bootloader
has not been used and is possibly incompatible with the Clicker2-STM32
linker script at configs/clicker2-stm32/scripts/flash.ld. Often code must
be built to execute at an offset in to FLASH when a bootloader is used.
Certainly that is the case for the ST-Micro DFU bootloader but I am not
aware of the requirements for use with the mikroBootloader.

JTAG has been used in the development of this board support. The
Clicker2-STM32 board offers a 2x5 JTAG connector. You may use Dupont
jumpers to connect this port to JTAG as described here:

https://www.mikroe.com/how-to-use-st-link-v2-with-clicker-2-for-stm32-a-detailed-walkthrough/
http://www.playembedded.org/blog/en/2016/02/06/mikroe-clicker-2-for-stm32-and-stlink-v2/

NOTE that the FLASH probably has read protection enabled locked. You may
need to follow the instructions at the second link to unlock it. You can
also use the STM32 ST-Link CLI tool on Windows to remove the read protection
using the -OB command:

$ ./ST-LINK_CLI.exe -c SN=53FF6F064966545035320387 SWD LPM
STM32 ST-LINK CLI v2.3.0
STM32 ST-LINK Command Line Interface

ST-LINK SN : 53FF6F064966545035320387
ST-LINK Firmware version : V2J24S4
Connected via SWD.
SWD Frequency = 4000K.
Target voltage = 3.2 V.
Connection mode : Normal.
Debug in Low Power mode enabled.
Device ID:0x413
Device family :STM32F40xx/F41xx

$ ./ST-LINK_CLI.exe -OB RDP=0
STM32 ST-LINK CLI v2.3.0
STM32 ST-LINK Command Line Interface

ST-LINK SN : 53FF6F064966545035320387
ST-LINK Firmware version : V2J24S4
Connected via SWD.
SWD Frequency = 4000K.
Target voltage = 3.2 V.
Connection mode : Normal.
Device ID:0x413
Device family :STM32F40xx/F41xx
Updating option bytes...
Option bytes updated successfully.

NOTE:
1. You can get the ST-Link Utilies here:
http://www.st.com/en/embedded-software/stsw-link004.html
2. The ST-LINK Utility command line interface is located at:
[Install_Directory]\STM32 ST-LINK Utility\ST-LINK Utility\ST-LINK_CLI.exe
3. You can get a summary of all of the command options by running
ST-LINK_CLI.exe with no arguments.
4. You can get the serial number of the ST-Link when from the information
window if you connect via the ST-Link Utility:

11:04:28 : ST-LINK SN : 53FF6F064966545035320387
11:04:28 : ST-LINK Firmware version : V2J24S4
11:04:28 : Connected via SWD.
11:04:28 : SWD Frequency = 100 KHz.
11:04:28 : Connection mode : Normal.
11:04:28 : Debug in Low Power mode enabled.
11:04:30 : Device ID:0x413
11:04:30 : Device family :STM32F40xx/F41xx
11:04:30 : Can not read memory!
Disable Read Out Protection and retry.

You can avoid the mess of jumpers using the mikroProg to ST-Link v2 adapter
along with a 2x5, 10-wire ribbon cable connector:

https://shop.mikroe.com/add-on-boards/adapter/mikroprog-st-link-v2-adapter

Then you can use the ST-Link Utility or other debugger software to write
the NuttX binary to FLASH. OpenOCD can be used with the ST-Link to provide
a debug environment. The debug adaptor is NOT compatible with other JTAG
debuggers such as the Segger J-Link.

Configurations
==============

Information Common to All Configurations
----------------------------------------
Each Clicker2 configuration is maintained in a sub-directory and can be
selected as follow:

cd tools
./configure.sh clicker2-stm32/
cd -

Before building, make sure the PATH environment variable includes the
correct path to the directory than holds your toolchain binaries.

And then build NuttX by simply typing the following. At the conclusion of
the make, the nuttx binary will reside in an ELF file called, simply, nuttx.

make oldconfig
make

The that is provided above as an argument to the tools/configure.sh
must be is one of the following.

NOTES:

1. These configurations use the mconf-based configuration tool. To
change any of these configurations using that tool, you should:

a. Build and install the kconfig-mconf tool. See nuttx/README.txt
see additional README.txt files in the NuttX tools repository.

b. Execute 'make menuconfig' in nuttx/ in order to start the
reconfiguration process.

2. Unless stated otherwise, all configurations generate console
output on USART3, channel 0) as described above under "Serial
Console". The relevant configuration settings are listed below:

CONFIG_STM32_USART3=y
CONFIG_STM32_USART3_SERIALDRIVER=y
CONFIG_STM32_USART=y

CONFIG_USART3_SERIALDRIVER=y
CONFIG_USART3_SERIAL_CONSOLE=y

CONFIG_USART3_RXBUFSIZE=256
CONFIG_USART3_TXBUFSIZE=256
CONFIG_USART3_BAUD=115200
CONFIG_USART3_BITS=8
CONFIG_USART3_PARITY=0
CONFIG_USART3_2STOP=0


3. All of these configurations are set up to build under Linux using the
"GNU Tools for ARM Embedded Processors" that is maintained by ARM
(unless stated otherwise in the description of the configuration).

https://developer.arm.com/open-source/gnu-toolchain/gnu-rm

That toolchain selection can easily be reconfigured using
'make menuconfig'. Here are the relevant current settings:

Build Setup:
CONFIG_HOST_LINUX =y : Linux environment

System Type -> Toolchain:
CONFIG_ARMV7M_TOOLCHAIN_GNU_EABIL=y : GNU ARM EABI toolchain

Configuration sub-directories
-----------------------------

knsh:

This is identical to the nsh configuration below except that NuttX
is built as a protected mode, monolithic module and the user applications
are built separately.

It is recommends to use a special make command; not just 'make' but make
with the following two arguments:

make pass1 pass2

In the normal case (just 'make'), make will attempt to build both user-
and kernel-mode blobs more or less interleaved. This actual works!
However, for me it is very confusing so I prefer the above make command:
Make the user-space binaries first (pass1), then make the kernel-space
binaries (pass2)

NOTES:

1. At the end of the build, there will be several files in the top-level
NuttX build directory:

PASS1:
nuttx_user.elf - The pass1 user-space ELF file
nuttx_user.hex - The pass1 Intel HEX format file (selected in defconfig)
User.map - Symbols in the user-space ELF file

PASS2:
nuttx - The pass2 kernel-space ELF file
nuttx.hex - The pass2 Intel HEX file (selected in defconfig)
System.map - Symbols in the kernel-space ELF file

The J-Link programmer will accept files in .hex, .mot, .srec, and .bin
formats. The St-Link programmer will accept files in hex and .bin
formats.

2. Combining .hex files. If you plan to use the .hex files with your
debugger or FLASH utility, then you may need to combine the two hex
files into a single .hex file. Here is how you can do that.

a. The 'tail' of the nuttx.hex file should look something like this
(with my comments added):

$ tail nuttx.hex
# 00, data records
...
:10 9DC0 00 01000000000800006400020100001F0004
:10 9DD0 00 3B005A0078009700B500D400F300110151
:08 9DE0 00 30014E016D0100008D
# 05, Start Linear Address Record
:04 0000 05 0800 0419 D2
# 01, End Of File record
:00 0000 01 FF

Use an editor such as vi to remove the 05 and 01 records.

b. The 'head' of the nuttx_user.hex file should look something like
this (again with my comments added):

$ head nuttx_user.hex
# 04, Extended Linear Address Record
:02 0000 04 0801 F1
# 00, data records
:10 8000 00 BD89 01084C800108C8110208D01102087E
:10 8010 00 0010 00201C1000201C1000203C16002026
:10 8020 00 4D80 01085D80010869800108ED83010829
...

Nothing needs to be done here. The nuttx_user.hex file should
be fine.

c. Combine the edited nuttx.hex and un-edited nuttx_user.hex
file to produce a single combined hex file:

$ cat nuttx.hex nuttx_user.hex >combined.hex

Then use the combined.hex file with the to write the FLASH image.
If you do this a lot, you will probably want to invest a little time
to develop a tool to automate these steps.

mrf24j40-mac

This is a version of nsh that was used for testing the MRF24J40 MAC be
as a character device. The most important configuration differences are
summarized below:

1. Support for the BEE click and SPI are in enabled in the mikroBUS1 slot:

CONFIG_CLICKER2_STM32_MB1_BEE=y
CONFIG_CLICKER2_STM32_MB1_SPI=y

2. SPI support and STM32 SPI3, in particular, are enabled:

CONFIG_SPI=y
CONFIG_SPI_EXCHANGE=y

CONFIG_STM32_SPI=y
CONFIG_STM32_SPI3=y

4. Support for the IEEE802.15.4 "upper half" character driver is enabled:

CONFIG_WIRELESS=y
CONFIG_WIRELESS_IEEE802154=y
CONFIG_IEEE802154_MAC_DEV=y
CONFIG_IEEE802154_NTXDESC=3
CONFIG_IEEE802154_IND_PREALLOC=20
CONFIG_IEEE802154_IND_IRQRESERVE=10
CONFIG_IEEE802154_DEFAULT_EADDR=0x00fade00deadbeef

5. Support for the lower half MRF24J40 character driver is enabled

CONFIG_DRIVERS_WIRELESS=y
CONFIG_DRIVERS_IEEE802154=y
CONFIG_IEEE802154_MRF24J40=y

6. Support for the i8sak test program at apps/ieee802154 is enabled:

CONFIG_IEEE802154_LIBMAC=y
CONFIG_IEEE802154_LIBUTILS=y
CONFIG_IEEE802154_I8SAK=y
CONFIG_IEEE802154_I8SAK_PRIORITY=100
CONFIG_IEEE802154_I8SAK_STACKSIZE=2048

7. Initialization hooks are provided to enable the MRF24J40 and to
register the radio character driver.

CONFIG_NSH_ARCHINIT=y

8. Configuration instructions: WPAN configuration must be performed
using the i8sak program. Detailed instructions are provided in a
README.txt file at apps/wireless/ieee802154/i8sak. You should make
sure that you are familiar with the content of that README.txt file.

Here is a quick "cheat sheet" for associated to setting up a
coordinator and associating wth the WPAN:

1. Configure the Coordinator. On coordinator device do:

nsh> i8 /dev/ieee0 startpan
nsh> i8 acceptassoc

2. Assocate an endpoint device with the WPAN. On the endpoint
device:

nsh> i8 /dev/ieee0 assoc

mrf24j40-6lowpan

This is another version of nsh that is very similar to the mrf24j40-mac
configuration but is focused on testing the IEEE 802.15.4 MAC
integration with the 6LoWPAN network stack. It derives directly from the
mrf24j40-mac and all NOTES provided there apply. Additional differences
are summarized below:

NOTES:

1. You must have two clicker2-stm32 boards each with an MRF24J40 click
board in order to run these tests.

2. This configuration differs from the mrf24j40-mac configuration in
that this configuration, like the usbnsh configuration, uses a USB
serial device for console I/O. Such a configuration is useful on the
Clicker2 STM32 which has no builtin RS-232 drivers and eliminates the
tangle of cables and jumpers needed to debug multi-board setups.

Most other NOTES for the usbnsh configuration should apply. Specific
differences between the usbnsh or mrf24j40-mac configurations and this
configuration are listed in these NOTES.

3. On most serial terminal programs that I have used, the USB
connection will be lost when the target board is reset. When that
happens, you may have to reset your serial terminal program to adapt
to the new USB connection. Using TeraTerm, I actually have to exit
the serial program and restart it in order to detect and select the
re-established USB serial connection.

4. This configuration does NOT have USART3 output enabled. This
configuration supports logging of debug output to a circular
buffer in RAM. This feature is discussed fully in this Wiki page:
http://nuttx.org/doku.php?id=wiki:howtos:syslog . Relevant
configuration settings are summarized below:

Device Drivers:
CONFIG_RAMLOG=y : Enable the RAM-based logging feature.
CONFIG_RAMLOG_CONSOLE=n : (We don't use the RAMLOG console)
CONFIG_RAMLOG_SYSLOG=y : This enables the RAM-based logger as the
system logger.
CONFIG_RAMLOG_NONBLOCKING=y : Needs to be non-blocking for dmesg
CONFIG_RAMLOG_BUFSIZE=8192 : Buffer size is 8KiB

NOTE: This RAMLOG feature is really only of value if debug output
is enabled. But, by default, no debug output is disabled in this
configuration. Therefore, there is no logic that will add anything
to the RAM buffer. This feature is configured and in place only
to support any future debugging needs that you may have.

If you don't plan on using the debug features, then by all means
disable this feature and save 8KiB of RAM!

NOTE: There is an issue with capturing data in the RAMLOG: If
the system crashes, all of the crash dump information will go into
the RAMLOG and you will be unable to access it! You can tell that
the system has crashed because (a) it will be unresponsive and (b)
the LD2 will be blinking at about 2Hz.

5. IPv6 networking is enabled with TCP/IP, UDP, 6LoWPAN, and NSH
Telnet support.

6. Configuration instructions: Basic PAN configuration is the same as
for the ieee802154-mac configuration with the exception that after
the PAN has been configured with the i8sak utility, you must
explicity bring the network up on each node:

nsh> ifup wpan0

7. examples/udp is enabled. This will allow two MRF24J40 nodes to
exchange UDP packets. Basic instructions:

On the server node:

nsh> ifconfig
nsh> udpserver &

The ifconfig command will show the IP address of the server. Then on
the client node use this IP address to start the client:

nsh> udpclient &

Where is the IP address of the server that you got above.
NOTE: There is no way to stop the UDP test once it has been started
other than by resetting the board.

Cheat Sheet. Here is a concise summary of all all the steps needed to
run the UDP test (C=Coordinator; E=Endpoint):

C: nsh> i8 /dev/ieee0 startpan
C: nsh> 8 acceptassoc
E: nsh> i8 assoc
C: nsh> ifup wpan0
C: nsh> ifconfig <-- To get the
E: nsh> ifup wpan0
C: nsh> udpserver &
E: nsh> udpclient &

The nsh> dmesg command can be use at any time on any node to see
any debug output that you have selected.

8. examples/nettest is enabled. This will allow two MRF24J40 nodes to
exchange TCP packets. Basic instructions:

On the server node:

nsh> ifconfig
nsh> tcpserver &

The ifconfig command will show the IP address of the server. Then on
the client node use this IP address to start the client:

nsh> tcpclient &

Where is the IP address of the server that you got above.
NOTE: Unlike the UDP test, there the TCP test will terminate
automatically when the packet exchange is complete.

Cheat Sheet. Here is a concise summary of all all the steps needed to
run the TCP test (C=Coordinator; E=Endpoint):

C: nsh> i8 /dev/ieee0 startpan
C: nsh> 8 acceptassoc
E: nsh> i8 assoc
C: nsh> ifup wpan0
C: nsh> ifconfig <-- To get the
E: nsh> ifup wpan0
C: nsh> tcpserver &
E: nsh> tcpclient &

The nsh> dmesg command can be use at any time on any node to see
any debug output that you have selected.

9. The NSH Telnet deamon (server) is enabled. However, it cannot be
started automatically. Rather, it must be started AFTER the network
has been brought up using the NSH 'telnetd' command. You would want
to start the Telent daemon only if you want the node to serve Telent
connections to an NSH shell on the node.

nsh> ifconfig
nsh> telnetd

Note the 'ifconfig' is executed to get the IP address of the node.
This is necessary because the IP address is assigned by the the
Coordinator and may not be known a priori.

10. This configuration also includes the Telnet client program. This
will allow you to execute a NSH one a node from the command line on
a different node. Like:

nsh> telnet

Where is the IP address of the server that you got for
the ifconfig commna on the remote node. Once the telnet session
has been started, you can end the session with:

nsh> exit

Cheat Sheet. Here is a concise summary of all all the steps needed to
run the TCP test (C=Coordinator; E=Endpoint):

C: nsh> i8 /dev/ieee0 startpan
C: nsh> 8 acceptassoc
E: nsh> i8 assoc
C: nsh> ifup wpan0
C: nsh> ifconfig <-- To get the
E: nsh> ifup wpan0
C: nsh> telnetd <-- Starts the Telnet daemon
E: nsh> telnet <-- Runs the Telnet client

STATUS:

2017-06-21: Basic UDP functionality has been achieved with HC06
compression and short address. Additional testing is required for
other configurations (see text matrix below).

2017-06-23: Added test for TCP functionality. As of yet unverified.

2017-06-24: There are significant problems with the 6LoWPAN TCP send
logic. A major redesign was done to better handle ACKs and
retransmissions, and to work with TCP dynamic windowing.

2017-05-25: After some rather extensive debug, the TCP test was made
to with (HC06 and short addressing).

2017-06-26: Verified with HC06 and extended addressing and HC1 with
both addressing modes.

2017-06-27: Added the Telnet client application to the configuration.
Initial testing reveal a problem that required re-design of the
Telnet daemon: It did not yet support IPv6! But after adding this
support, Telnet worked just fine.

Test Matrix:
The following configurations have been tested:

TEST DATE
COMPRESSION ADDRESSING UDP TCP
----------- ---------- ---- ----
hc06 short 6/21 6/25
extended 6/22 6/26
hc1 short 6/23 6/26
extended 6/23 6/26
ipv6 short --- ---
extended --- ---
telnet short N/A 6/27 (hc06)
extended N/A ---

Other configuration options have not been specifically addressed
(such non-compressable ports, non-MAC based IPv6 addresses, etc.)

One limitation of this test is that it only tests NuttX 6LoWPAN
against NuttX 6LoWPAN. It does not prove that NuttX 6LoWPAN is
compatible with other implementations of 6LoWPAN. The tests could
potentially be verifying only that the design is implemented
incorrectly in compatible way on both the client and server sides.

mrf24j40-starhub and mrf24j40-starpoint

These two configurations implement hub and and star endpoint in a
star topology. Both configurations derive from the mrf24j40-6lowpan
configuration and most of the notes there apply here as well.

1. You must have three clicker2-stm32 boards each with an MRF24J40
click board in order to run these tests: One that serves as the
star hub and at least two star endpoints.

2. The star point configuration differs from the primarily in the
mrf24j40-6lowpan in following is also set:

CONFIG_NET_STAR=y
CONFIG_NET_STARPOINT=y

The CONFIG_NET_STARPOINT selection informs the endpoint that it
must send all frames to the hub of the star, rather than directly
to the recipient.

The star hub configuration, on the other hand, differs from the
mrf24j40-6lowpan in these fundamental ways:

CONFIG_NET_STAR=y
CONFIG_NET_STARHUB=y
CONFIG_NET_IPFORWARD=y

The CONFIG_NET_IPFORWARD selection informs the hub that if it
receives any packets that are not destined for the hub, it should
forward those packets appropriately.

3. Telnet: The star point configuration supports the Telnet daemon,
but not the Telnet client; the star hub configuration supports
the Telnet client, but not the Telnet daemon. Therefore, the
star hub can Telnet to any point in the star, the star endpoints
cannot initiate telnet sessions.

4. TCP and UDP Tests: The same TCP and UDP tests as described for
the mrf24j40-6lowpan coniguration are supported on the star
endpoints, but NOT on the star hub. Therefore, all network testing
is between endpoints with the hub acting, well, only like a hub.

The modified usage of the TCP test is show below with E1 E2
representing the two star endpoints and C: representing the
coordinator/hub.

C: nsh> i8 /dev/ieee0 startpan
C: nsh> 8 acceptassoc
E1: nsh> i8 assoc
E2: nsh> i8 assoc
C: nsh> ifup wpan0
E1: nsh> ifup wpan0
E1: nsh> ifconfig <-- To get the IP address of E1 endpoint
E1: nsh> telnetd <-- Starts the Telnet daemon
E2: nsh> ifup wpan0
E2: nsh> ifconfig <-- To get the IP address of E2 endpoint
E2: nsh> telnetd <-- Starts the Telnet daemon
E1: nsh> tcpserver &
E2: nsh> tcpclient &

Where is the IP address of the E1 endpoint.

Similarly for the UDP test:

E1: nsh> udpserver &
E2: nsh> udpclient &

The nsh> dmesg command can be use at any time on any node to see
any debug output that you have selected.

Telenet sessions may be initiated only from the hub to a star
endpoint:

C: nsh> telnet <-- Runs the Telnet client

Where is the IP address of either the E1 or E2 endpoints.

STATUS:
2017-06-29: Configurations added. Initial testing indicates that
the TCP Telnet client can successfully establish sessions with
the two star endpoints. When testing communications between the
two star endpoints via the hub, the frames are correctly directed
to the hub. However, they are not being forwarded to the other
endpoint.
2017-06-30: The failure to forward is understood: When the star
endpoint sent the IPv6 destination address, the HC06 compression
logic elided the address -- meaning that it could be reconstructed
based on the receiver's assigned short address. However, when
intercepted by the hub, the uncompressed address does not know
the short address of the recipient and instead uses the short
address of the hub. This means two things: (1) it looks like
the hub address is the destination address, and (2) the
uncompressed UDP packet has a bad checksum.

This required a change to assure that the destination IPv6 address
is not elided in the case of the star endpoint configuration. After
some additional fixes for byte ordering in 16-bit and 64-bit
compressed IPv6 addresses, then all tests are working as expected:
TCP, UDP, Telnet.
2017-08-5: It looks like I have lost one of my Clicker2-STM32 boards.
This means that I will not be able to do any regression testing as
changes are made to the radio interfaces and 6LoWPAN :(

nsh:

Configures the NuttShell (nsh) located at examples/nsh. This
configuration is focused on low level, command-line driver testing. It
has no network.

NOTES:

1. Support for NSH built-in applications is provided:

Binary Formats:
CONFIG_BUILTIN=y : Enable support for built-in programs

Application Configuration:
CONFIG_NSH_BUILTIN_APPS=y : Enable starting apps from NSH command line

No built applications are enabled in the base configuration, however.

2. C++ support for applications is enabled:

CONFIG_HAVE_CXX=y
CONFIG_HAVE_CXXINITIALIZE=y
CONFIG_EXAMPLES_NSH_CXXINITIALIZE=y

usbnsh:

This is another NSH example. If differs from other 'nsh' configurations
in that this configurations uses a USB serial device for console I/O.
Such a configuration is useful on the Clicker2 STM32 which has no
builtin RS-232 drivers.

NOTES:

1. One most serial terminal programs that I have used, the USB
connection will be lost when the target board is reset. When that
happens, you may have to reset your serial terminal program to adapt
to the new USB connection. Using TeraTerm, I actually have to exit
the serial program and restart it in order to detect and select the
re-established USB serial connection.

2. This configuration does have USART3 output enabled and set up as
the system logging device:

CONFIG_SYSLOG_CHAR=y : Use a character device for system logging
CONFIG_SYSLOG_DEVPATH="/dev/ttyS0" : USART3 will be /dev/ttyS0

However, there is nothing to generate SYLOG output in the default
configuration so nothing should appear on USART3 unless you enable
some debug output or enable the USB monitor.

3. Enabling USB monitor SYSLOG output. If tracing is enabled, the USB
device will save encoded trace output in in-memory buffer; if the
USB monitor is enabled, that trace buffer will be periodically
emptied and dumped to the system logging device (USART3 in this
configuration):

CONFIG_USBDEV_TRACE=y : Enable USB trace feature
CONFIG_USBDEV_TRACE_NRECORDS=128 : Buffer 128 records in memory
CONFIG_NSH_USBDEV_TRACE=n : No builtin tracing from NSH
CONFIG_NSH_ARCHINIT=y : Automatically start the USB monitor
CONFIG_USBMONITOR=y : Enable the USB monitor daemon
CONFIG_USBMONITOR_STACKSIZE=2048 : USB monitor daemon stack size
CONFIG_USBMONITOR_PRIORITY=50 : USB monitor daemon priority
CONFIG_USBMONITOR_INTERVAL=2 : Dump trace data every 2 seconds

CONFIG_USBMONITOR_TRACEINIT=y : Enable TRACE output
CONFIG_USBMONITOR_TRACECLASS=y
CONFIG_USBMONITOR_TRACETRANSFERS=y
CONFIG_USBMONITOR_TRACECONTROLLER=y
CONFIG_USBMONITOR_TRACEINTERRUPTS=y

Using the Prolifics PL2303 Emulation
------------------------------------
You could also use the non-standard PL2303 serial device instead of
the standard CDC/ACM serial device by changing:

CONFIG_CDCACM=n : Disable the CDC/ACM serial device class
CONFIG_CDCACM_CONSOLE=n : The CDC/ACM serial device is NOT the console
CONFIG_PL2303=y : The Prolifics PL2303 emulation is enabled
CONFIG_PL2303_CONSOLE=y : The PL2303 serial device is the console