Difference between revisions of "MTB specifics"
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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. |
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. |
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− | = |
+ | = Features = |
− | MTB is a |
+ | MTB is a stripped down version of trace that has the following features: |
− | * Instruction |
+ | * Instruction backtrace |
+ | |||
− | * No streaming trace possible |
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+ | The following is NOT supported by MTB |
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− | * A portion of the target RAM may not be used by the application if MTB is used |
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+ | * Streaming trace |
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+ | * Complete code coverage |
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= RAM usage = |
= RAM usage = |
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− | Most MTBs have a trace buffer between 2 |
+ | 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). |
By default, J-Link uses 512 bytes for MTB, starting at the MTB_BASE addr (device specific). |
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+ | |||
+ | '''Note:''' The RAM assigned to the MTB must not be used by the application while MTB tracing is active |
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= Configuration options provided by J-Link = |
= Configuration options provided by J-Link = |
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* J-Link can detect the max. RAM usage of MTB via the MTB_POINTER register |
* J-Link can detect the max. RAM usage of MTB via the MTB_POINTER register |
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− | Target RAM = 0x20000000-0x20005FFF (24 |
+ | Target RAM = 0x20000000-0x20005FFF (24 KiB)<br> |
MTB_BASE = 0x20004000<br> |
MTB_BASE = 0x20004000<br> |
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MTB max. RAM usage = 0x2000 |
MTB max. RAM usage = 0x2000 |
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− | Valid configurations |
+ | === Valid configurations === |
*MTB use: 0x20004000 - 0x200041FF (Default. CORESIGHT_SetMTBBufUseAddr: 0x20004000, CORESIGHT_SetMTBBufUsageSize: 0x200) |
*MTB use: 0x20004000 - 0x200041FF (Default. CORESIGHT_SetMTBBufUseAddr: 0x20004000, CORESIGHT_SetMTBBufUsageSize: 0x200) |
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*MTB use: 0x20004000 - 0x2000401F (CORESIGHT_SetMTBBufUseAddr: 0x20004000, CORESIGHT_SetMTBBufUsageSize: 0x10) |
*MTB use: 0x20004000 - 0x2000401F (CORESIGHT_SetMTBBufUseAddr: 0x20004000, CORESIGHT_SetMTBBufUsageSize: 0x10) |
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*MTB use: 0x20004100 - 0x2000417F (CORESIGHT_SetMTBBufUseAddr: 0x20004100, CORESIGHT_SetMTBBufUsageSize: 0x80) |
*MTB use: 0x20004100 - 0x2000417F (CORESIGHT_SetMTBBufUseAddr: 0x20004100, CORESIGHT_SetMTBBufUsageSize: 0x80) |
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− | Invalid configurations |
+ | === Invalid configurations === |
− | MTB use: 0x20004000 - 0x20004017 (UsageSize = 0x18 which is not a power of 2) |
+ | *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) |
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− | |||
− | 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) |
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= MTB internals = |
= MTB internals = |
Latest revision as of 09:02, 27 May 2021
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.
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 |
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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
- 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)