MTB specifics

The ARM Micro Trace Buffer (MTB) is an on-chip trace buffer that allows to do post-mortem instruction trace via a regular J-Link with no need of a specific J-Trace unit. The MTB will use a specific amount of the on-chip RAM to store trace data while the core executes instructions. MTB is mainly used on smaller MCUs like Cortex-M0+/M23/M33 based ones.

Limitations

MTB is a very limited version of trace that has the following limitations:

• Instruction trace support only
• No streaming trace possible
• A portion of the target RAM may not be used by the application if MTB is used

RAM usage

Most MTBs have a trace buffer between 2 KB and 8 KB. By default, J-Link uses 512 bytes for MTB, starting at the MTB_BASE addr.

Configuration options provided by J-Link

An MTB consists of the following registers / bit fields that allow configuration of the MTB and the amount of RAM it uses:

Register / Bit field	Description
MTB_BASE	32-bit register that specifies the base address of the MTB buffer
MTB_POINTER	32-bit register that specifies the offset (rel. to BASE) where the next trace packet will be stored by the HW
MTB_MASTER.MASK	5-bit bitfield that specifies the wrap-around point for the auto-inc logic of the MTB, on packet store.

While the MTB_BASE register is read-only for J-Link, the MTB_POINTER and MTB_MASTER.MASK can be written by J-Link. This allows the MTB RAM usage to be configured.

MTB_MASTER.MASK bit field

Specifies the wrap-around point for the auto-inc logic of the MTB, on packet store. MASK specifies the MSB that exists in the buffer. A MASK Value of 7 means that bits 0-7 exist in the buffer, which means a max. value (buffer size) of 0xFF + 1 = 0x100. If the increment causes POINTER[MASK:MASK] to change from 1 to 0, POINTER[MASK:0] will be changed to 0, while leaving the rest of POINTER untouched(!). The MASK value is stored as MASK - 4 because the MTB buffer size has to be a multiple of 16 bytes anyhow.

Example calculation:

• 1 KB of RAM shall be used
• 1 KB = 0x400 (MSB == bit 10, counting starts at bit 0)
• MASK = 9 (Stored in register as: 10 - 4 = 6)
• Buffer usage: 0x0 - 0x3FF

Default J-Link config

1. J-Link reads out MTB_BASE to determine the base addr. of the MTB buffer
2. J-Link writes the POINTER register to find out the MTB buffer size
3. J-Link sets the POINTER register to 0 to start using the MTB buffer at MTB_BASE
4. J-Link sets the MTB_MASTER.MASK to 4 to provide a 512 byte wrap-around (setting the RAM usage to 512 bytes, starting at MTB_BASE)

Command strings for MTB configuration

The following J-Link command strings are available for MTB configuration:

Register / Bit field	Min. J-Link software version	Description
hhh	hhh	hhh
hhh	hhh	hhh
hhh	hhh	hhh

For more information about how to use J-Link command strings, please refer to Using J-Link Command Strings

Example configurations

TBD