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SWO and variables reading

Note: This was originally posted on 27th February 2012 at http://forums.arm.com

Dear all,

I would like to create a SWV application, and my question is the following: how many variables can I read via SWO communication?

I mean, without considering the overflow packet that could be sent in case of huge information traffic, is there a physical limit regarding this topic?

If I've understood weel, DWT_COMPx registers, with x={1, .., 4} is used in order to store the variable address that I would like to monitor, and DWT_MASKx are also used in order to raise a valid new SWO packet.

Does this mean that it's possible to check only for four variables, or should I set the mask register in order to detect ranges of variables?
With the latter case I could be able to monitor a very large number of items.

Am I wrong or is this the right approach?

Thank you very much!

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Joseph Yiu over 12 years ago
Note: This was originally posted on 23rd March 2012 at http://forums.arm.com

Hi,

Regarding DWTx_MASK:
0 = 1 byte
1 = 2 bytes
2 = 4 bytes
3 = 8 bytes
4 = 16 bytes , ..., etc

> DWT_COMPx = address of the variable to monitor (if several variables are contiguously defined, then here the lower address)

Please note that you need to have the address value being multiple of the compare address size. For example, if DWTx_MASK is set to 4 (16 bytes), then the address value should be multiple of 16.

You can use DWT to halt the processor when the PC match a certain value. In that case you need to set the Func field in DWTx_FUNC to 0x4.
But for monitoring variables using SWO you use function 1 - 3, and 12-15, with DWTx_COMP being the address value of the variable(s).

You can setup the DWTx_MASK to capture multiple variables in an address range, but by doing so you will need to emit the address values as well so that you can tell which variable of that address range is being accessed. Also, if you try to capture a large address range you can easily get buffer overflow in the trace system.

In most cases, since there are only maximum of 4 comparators in DWT, a debugger will limit the number of software variables that you can monitor to 4 to make it easier to handle.

If you want to generate a packet when a byte in 0x2000017C is written to, then
DWT_CTRL = 0x400003FE
DWT_COMP0 = 0x2000017C (address to monitor)
DWT_MASK0 = 0x0000000 (1 byte to monitor)
DWT_FUNCTION0 = 0x0000000D (emit data for write transfer) / 0x0000000E (emit PC & data for write transfer)
regards,
Joseph
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Joseph Yiu over 12 years ago
Note: This was originally posted on 23rd March 2012 at http://forums.arm.com

Hi,

Regarding DWTx_MASK:
0 = 1 byte
1 = 2 bytes
2 = 4 bytes
3 = 8 bytes
4 = 16 bytes , ..., etc

> DWT_COMPx = address of the variable to monitor (if several variables are contiguously defined, then here the lower address)

Please note that you need to have the address value being multiple of the compare address size. For example, if DWTx_MASK is set to 4 (16 bytes), then the address value should be multiple of 16.

You can use DWT to halt the processor when the PC match a certain value. In that case you need to set the Func field in DWTx_FUNC to 0x4.
But for monitoring variables using SWO you use function 1 - 3, and 12-15, with DWTx_COMP being the address value of the variable(s).

You can setup the DWTx_MASK to capture multiple variables in an address range, but by doing so you will need to emit the address values as well so that you can tell which variable of that address range is being accessed. Also, if you try to capture a large address range you can easily get buffer overflow in the trace system.

In most cases, since there are only maximum of 4 comparators in DWT, a debugger will limit the number of software variables that you can monitor to 4 to make it easier to handle.

If you want to generate a packet when a byte in 0x2000017C is written to, then
DWT_CTRL = 0x400003FE
DWT_COMP0 = 0x2000017C (address to monitor)
DWT_MASK0 = 0x0000000 (1 byte to monitor)
DWT_FUNCTION0 = 0x0000000D (emit data for write transfer) / 0x0000000E (emit PC & data for write transfer)
regards,
Joseph
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