Keil debugging techniques and alternative printf (SWO function)

One of the basic needs of the embedded software development through the terminal to output debugging information, generally two ways to achieve:

one is the COM port on the UART and PC using a serial cable connecting plate through the PC HyperTerminal to view the debugging information;

Another mechanism is the use of semi-host, but it may not be used in tool chain support.

Cortex-M3 core-based software debug break this limit, the Cortex-M3 core provides

a ITM (Instrumentation TraceMacrocell) interface debugging theITM data received by the SWO pin SWV (Serial Wire Viewer).

ITM 32 common data channel, based on the implementation the CMSIS requirements as the debugging output terminal to output debugging information,

channel 31 for the operating system (privileged mode access) channel 0.

In core_cm3 h the defined ITM_SendChar () function by calling the function to rewrite fputc print debugging information in the application by printf,

these debugging information can be viewed by the ITM Viewer.

With this implementation, embedded software developers can not configure the serial port

and use the terminal debugging software output debugging information to some extent reduce the workload. 
 
Basic concepts:

SWD 
The J-Link and J-Trace support ARMs Serial WireDebug (SWD).

SWD replaces the 5-pin JTAG port with a clock (SWDCLK) and a single bi-directional data pin (SWDIO),

providing all thenormal JTAG debug and testfunctionality.

Pin 13 of SWD: 
SWO - Serial Wire Output trace port. (Optional, not required for SWD communication.) 
 
SWO 
J-Link can be used with devices that supportSerial Wire Output (SWO).

Serial Wire Output (SWO) support meanssupport for a single pin output signal from the core. It iscurrently tested with Cortex-M3 only. 
 
SWV 
The Instrumentation Trace Macrocell (ITM) andSerial Wire Output (SWO) can be used to form a Serial Wire Viewer (SWV).

The Serial Wire Viewer provides a low cost method ofobtaining information from inside the MCU.

The SWO can output tracedata in two output formats, but only one output mechanism is validat any one time.

The 2 defined encodings are UART and Manchester.

The current J-Link implementation only supports UART encoding.

Serial Wire Viewer uses the SWO pin to transmit different packetsfor different types of information.

The three sources in theCortex-M3 core which can output information via this pinare:

- Instrumentation Trace Macrocell (ITM) forapplication-driven trace source that supports printf-styledebugging.
It supports 32 different channels, which allow it to beused for other purposes such as real-time kernel information aswell.

- Data Watchpoint and Trace (DWT) for real-timevariable monitoring and PC-sampling,
which can in turn be used toperiodically output the PC or various CPU-internal counters,
whichcan be used to obtain profiling information from thetarget.

- Timestamping. Timestamps are emitted relative topackets. 
 Debug (printf) Viewer

Home »?Vision Windows >> Debug (printf) Viewer 
The Debug (printf) Viewer window displays data streams that are transmitted sequentially through the ITM Stimulus Port 0. Enable ITM Stimulus Port 0.

Debug Viewer window

To use the Debug (printf) Viewer for tracing:

1. Add ITM Port register definitions to your source code.

# Define ITM_Port8 (n) (* ((volatile unsigned char *) (0xE0000000 + * n)))
# Define ITM_Port16 (n) (* ((volatile unsigned short *) (0xE0000000 + * n)))
# Define ITM_Port32 (n) (* ((volatile unsigned long *) (0xE0000000 + * n))) 
 
# Define DEMCR (* ((volatile unsigned long *) (0xE000EDFC)))
# Define TRCENA 0x01000000 

2. Add an fputc function to your source code that writes to the ITM Port 0 register. The fputc function enables printf to output messages.

struct __ FILE
{ int handle; / * Add whatever you need here * /};
FILE __ stdout;
FILE __ stdin;
 
int fputc( int ch, FILE * f )
{
  if ( DEMCR & TRCENA )
  {
    the
    while ( ITM_Port32(  ) ==  )
      ;
    ITM_Port8(  ) = CH;
  }
  return ( ch );
}

3.Add your debugging trace messages to your source code using printf.

printf ("AD value = 0x% 04X \ r \ n", AD_value); 

4.Set the ITM Port 0 to capture the information. Clear the Port 7 .. 0 privilege bit to access ITM Port 0 from User mode.

ITM Stimulus Port 0 
Open the View - Serial Windows - Debug (printf) Viewer window.

Note

ITM Stimulus Ports can be monitored in the Instruction Trace Window, where ITM Port 0 is shown as well. 
Consult Configure Cortex-M Target of the MDK-Primer for information on how to retarget the output.