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UsingSWO: Revisit Baudrate calculation and limits. 

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
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
a maximum speed of 4.5Mbps. For continious streaming that turns out to be
_too_ fast for the USB 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
link. The UART baudrate is set by b=(72x10^6)/d...with d >= 16 or
a maximum speed of 4.5Mbps UART1 and 2.25 Mbps on UART2.
For continious streaming that turns out to be_too_ fast for the USB
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
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
the it is a pretty long run before it becomes a problem).
Note that the baudrate equation means there are only certain speeds
available. The highest half dozen are;
SWO uses USART1(stlink) USART2(swlink)
1 4.50 Mbps 2.25 Mbps
2 2.25 Mbps 1.125 Mbps
3 1.50 Mbps 0.75 Mbps
4 1.125 Mbps 0.5635 Mbps
5 0.900 Mbps 0.45 Mbps
6 0.750 Mbps 0.375 Mbps
available. The highest:
BRR USART1(stlink) USART2(swlink)
16 4.50 Mbps 2.25 Mbps
17 4.235 Mbps 2.118 Mbps
18 4.000 Mbps 2.0 Mbps
19 3.789 Mbps 1.895 Mbps
20 3.600 Mbps 1.8 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
close to these values as you can. As the speed comes down the spread between