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UsingSWO: Revisit Baudrate calculation and limits.

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There are more SWO frequency options than the old documents indicated
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Uwe Bonnes 2021-02-16 22:23:19 +01:00 committed by UweBonnes
parent 0df44e205b
commit aeae9f7cde
1 changed files with 16 additions and 12 deletions
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UsingSWO
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@ -14,24 +14,28 @@ monitor traceswo 115200
We are constrained on maximum input speed by both the capabilities of the We are constrained on maximum input speed by both the capabilities of the
BMP STM32F103 USART and the ability to get the packets back out over the USB BMP STM32F103 USART and the ability to get the packets back out over the USB
link. The UART baudrate is set by b=(72x10^6)/(16*d)...so for d=1 that means link. The UART baudrate is set by b=(72x10^6)/d...with d >= 16 or
a maximum speed of 4.5Mbps. For continious streaming that turns out to be a maximum speed of 4.5Mbps UART1 and 2.25 Mbps on UART2.
_too_ fast for the USB link, so the next available option is the 2.25Mbps For continious streaming that turns out to be_too_ fast for the USB
that we use. ....you can safely use the 4.5Mbps setting if your debug data link, so the next available option is the 2.25Mbps that we use. ....
You can safely use the 4.5Mbps setting if your debug data
is bursty, or if you're using a different CPU to the STM32F103 as your BMP is bursty, or if you're using a different CPU to the STM32F103 as your BMP
host, but you potentially run the risk of losing packets if you have long host, but you potentially run the risk of losing packets if you have long
runs of sending which the usb cannot flush in time (there's a 12K buffer, so runs of sending which the usb cannot flush in time (there's a 12K buffer, so
the it is a pretty long run before it becomes a problem). the it is a pretty long run before it becomes a problem).
Note that the baudrate equation means there are only certain speeds Note that the baudrate equation means there are only certain speeds
available. The highest half dozen are; available. The highest:
SWO uses USART1(stlink) USART2(swlink) BRR USART1(stlink) USART2(swlink)
1 4.50 Mbps 2.25 Mbps 16 4.50 Mbps 2.25 Mbps
2 2.25 Mbps 1.125 Mbps 17 4.235 Mbps 2.118 Mbps
3 1.50 Mbps 0.75 Mbps 18 4.000 Mbps 2.0 Mbps
4 1.125 Mbps 0.5635 Mbps 19 3.789 Mbps 1.895 Mbps
5 0.900 Mbps 0.45 Mbps 20 3.600 Mbps 1.8 Mbps
6 0.750 Mbps 0.375 Mbps ...
24 3.0 Mbps 1.5 Mbps
...
36 2.0 Mbps 1.0 Mbps
...the USART will cope with some timing slip, but it's advisible to stay as ...the USART will cope with some timing slip, but it's advisible to stay as
close to these values as you can. As the speed comes down the spread between close to these values as you can. As the speed comes down the spread between