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.

Features

MTB is a stripped down version of trace that has the following features:

• Instruction backtrace

The following is NOT supported by MTB

• Streaming trace
• Complete code coverage

RAM usage

Most MTBs have a trace buffer between 2 KiB and 8 KiB.

By default, J-Link uses 512 bytes for MTB, starting at the MTB_BASE addr (device specific).

Note: The RAM assigned to the MTB must not be used by the application while MTB tracing is active

Configuration options provided by J-Link

Command strings for MTB configuration

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

Command string	Min. J-Link software version	Description
CORESIGHT_SetMTBBaseAddr	V5.10m	Specifies where to find the MTB in the debug address space. Only needed in case the device provides incorrect CoreSight info (like the NXP LPC84x series)
CORESIGHT_SetMTBBufBaseAddr	V6.20g	Specifies the base address of the MTB buffer (RAM that may be used by MTB). This is only needed for devices where the MTB_BASE register returns an incorrect address. (E.g. on the NXP KL26Z series devices)
CORESIGHT_SetMTBBufUseAddr	V6.33i	Specifies where the MTB shall start using the RAM. This address may be greater than MTB_BASE but must be within the bound of MTB_BASE and MTB_BASE + MaxBufSize
CORESIGHT_SetMTBBufUsageSize	V6.33i	Specifies max. RAM usage for MTB (must be a power of 2). This must not exceed MaxBufSize

Notes:

• For more information about how to use J-Link command strings, please refer to Using J-Link Command Strings
• All command strings from above may only be passed after a connection to the target has been established successfully

Example configurations

In the following, some example configurations for different MTB configurations are given. The examples assume that:

• J-Link can auto-discover the MTB component in the debug address space
• J-Link can detect the base address of the MTB buffer via MTB_BASE register
• J-Link can detect the max. RAM usage of MTB via the MTB_POINTER register

Target RAM = 0x20000000-0x20005FFF (24 KiB)
MTB_BASE = 0x20004000
MTB max. RAM usage = 0x2000

Valid configurations

• MTB use: 0x20004000 - 0x200041FF (Default. CORESIGHT_SetMTBBufUseAddr: 0x20004000, CORESIGHT_SetMTBBufUsageSize: 0x200)
• MTB use: 0x20004000 - 0x2000401F (CORESIGHT_SetMTBBufUseAddr: 0x20004000, CORESIGHT_SetMTBBufUsageSize: 0x10)
• MTB use: 0x20004100 - 0x2000417F (CORESIGHT_SetMTBBufUseAddr: 0x20004100, CORESIGHT_SetMTBBufUsageSize: 0x80)

Invalid configurations

• MTB use: 0x20004000 - 0x20004017 (UsageSize = 0x18 which is not a power of 2)
• MTB use: 0x20004010 - 0x2000402F (UsageSize = 0x20 which is O.K., but buffer start address is not aligned to usage size. Next possible start addresses for a UsageSize of 0x20: 0x20004000 and 0x20004020)

MTB internals

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 read-only register that specifies the base address of the MTB buffer
MTB_POINTER	32-bit R/W register that specifies the offset (rel. to BASE) where the next trace packet will be stored by the HW
MTB_MASTER.MASK	5-bit R/W bitfield that specifies the wrap-around point for MTB buffer.

MTB_MASTER.MASK bit field

Specifies the wrap-around (buffer size) for the MTB buffer to be used. Only power of 2 wrap-around options are supported. The actual value stored in HW is MASK - 4 because 16 bytes is the minimum buffer size anyhow.

C-like logic for this field is as follows:

Bit = (POINTER >> MASK) & 1;
v = POINTER + 8;
BitAfter = (POINTER >> MASK) & 1;
if (Bit == BitAfter) {
  POINTER = v;
} else {
  v = (1 << MASK) - 1;               // Wrap-around
  POINTER = ~v;
}

Initial:
  BASE    = 0x20000000
  POINTER = 0x00000000
  MASK    = 8          => MSB[8] => 0x100
Trace session, POINTER change:
  0x00
  0x08
  0x10
  0x18
  0x20
  0x28
  ...
  0xF0
  0xF8 => increment to 0x100 not possible due to MASK. Therefore, POINTER wrapped around
  0x00
  0x08

Initial:
  BASE    = 0x20000000
  POINTER = 0x00000F00
  MASK    = 8          => MSB[8] => 0x100
Trace session, POINTER change:
  0xF00
  0xF08
  0xF10
  0xF18
  0xF20
  0xF28
  ...
  0xFF0
  0xFF8 => increment to 0x1000 not possible due to MASK. Therefore, POINTER wrapped around
  0xF00
  0xF08

Default behavior of J-Link

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)