MTB specifics
Contents
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
- J-Link reads out MTB_BASE to determine the base addr. of the MTB buffer
- J-Link writes the POINTER register to find out the MTB buffer size
- J-Link sets the POINTER register to 0 to start using the MTB buffer at MTB_BASE
- 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:
| Command string | Min. J-Link software version | Description |
|---|---|---|
| CORESIGHT_SetMTBBaseAddr | V5.10m and later | Specifies where to find the MTB in the debug address space. Only needed in case the ROM table of the device is defect and does not allow auto-discovery of the MTB component |
| CORESIGHT_SetMTBBufBaseAddr | V6.20g and later | 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 and later | 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 + MaxBufSize - 16 |
| CORESIGHT_SetMTBBufUsageSize | V6.33i and later | Specifies max. RAM usage for MTB. This must not exceed the max. usage size indicated by MTB_POINTER |
For more information about how to use J-Link command strings, please refer to Using J-Link Command Strings
Example configurations
TBD
