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Real Time Transfer (RTT)

lbuild module: modm:platform:rtt

This module implements the RTT protocol compatible with OpenOCD. RTT works by placing multiple ring-buffers in RAM which the debugger read and writes using background memory accesses via SWD. It therefore works on any Cortex-M devices without extra pins except for the SWDCLK and SWDIO.


The RTT channels are exposed as a UART interface which you can use like so:

modm::platform::Rtt rtt(/* channel= */0);
// Access data directly via UART interface: eg. loop back
if (uint8_t data; { rtt.write(data); }
// Or wrap into an IOStream for printing
                            modm::IOBuffer::DiscardIfFull> rtt_device(rtt);
modm::IOStream stream(rtt_device);
stream << "Hello World" << modm::endl;
// Reading is more annoying
char data;
if (data != modm::IOStream::eof) { /* process new data */ }

You can define the number of channels and their buffer size by setting the buffer.tx and buffer.rx set options. Note that you can define multiple buffer sizes indexed by channel. Here is an example of three channels:

<!-- Channel0: TX only with 256B buffer -->
<!-- Channel1: RX only with 128B buffer -->
<!-- Channel2: TX with 512B and RX with 64B buffer -->
<option name="modm:platform:rtt:buffer.tx">256, 0, 512</option>
<option name="modm:platform:rtt:buffer.rx">0, 128, 64</option>

You can set the buffer size to 0 if you don't want to use this channel direction. This won't allocate a buffer and save a little RAM.

Accessing Data

OpenOCD has built-in support for RTT and modm generates a config that opens each RTT channel as a TCP port starting at 9090 (ie. 9090=channel 0, 9091=channel 1, etc).

openocd -f modm/openocd.cfg -c modm_rtt

You can also call this from inside GDB via the monitor command:

(gdb) monitor modm_rtt
rtt: Searching for control block 'SEGGER RTT'
rtt: Control block found at 0x20001024
Listening on port 9090 for rtt connections
(gdb) continue&

You can then use for example telnet 9090 to connect to the stream.

Note that this connection does not halt the target, you should therefore be able to use this at any point during program execution.

A simple telnet client is integrated into the build system generator, however, it can only connect to one stream at a time (disconnect with Ctrl+D).

 $ scons log-rtt
╭───OpenOCD───> Real Time Transfer
╰─────RTT────── stm32f103rbt
Info : rtt: Searching for control block 'SEGGER RTT'
Info : rtt: Control block found at 0x20000008
Listening on port 9090 for rtt connections
loop 51
loop 52
loop 53

 $ make log-rtt channel=0
Info : rtt: Searching for control block 'SEGGER RTT'
Info : rtt: Control block found at 0x20000008
Listening on port 9090 for rtt connections
loop 58
loop 60
loop 61

If you want to use this as a proper communication channel with a custom protocol you should implement the OpenOCD config yourself (with different ports).

You can also use JLink to access the RTT data, which may be significantly faster than OpenOCD if the debug probe has hardware support for the protocol.

 $ scons log-rtt-jlink
╭────JLink────> Real Time Transfer
╰─────RTT────── stm32l476rgt6

SEGGER J-Link V7.84f - Real time terminal output
RTT Demo on Nucleo-64
loop: 0
loop: 1
loop: 2
loop: 3
loop: 4
loop: 5
loop: 6
loop: 7

This module is only available for rp, sam, stm32.



Receive buffer sizes

Default: [0]
Inputs: [0 ... 64Ki]


Transmit buffer sizes

Default: [512]
Inputs: [0 ... 64Ki]


modm:platform:rtt modm_platform_rtt modm: platform: rtt modm_architecture_uart modm: architecture: uart modm_platform_rtt->modm_architecture_uart

Limited availability: Check with 'lbuild discover' if this module is available for your target!