Serial driver and gap between each character

[baoshi]

Hi,
I'm testing Serial driver performance.
A simple program only calls
sdWrite(&SD1, buf, 64);
chThdSleepMilliseconds(500);
in the main loop.
SD1 is set to 115200 and 921600 respectively. All ChibiOS debug checks are disabled.
As can be seen from the attached screenshot.

For 115200 baud, there is a 4.54us gap in between two characters.
While for 921600 baud, the gap increased to 19.62us
I understand the gap may be caused by lock/unlock, signaling in the ISR, but why is so that it varies so much for different baud rate?

Thanks

[Giovanni]

Hi,

It is caused by the IRQ service times, if you need to operate at very high speeds then try the UART driver, it uses DMA.

Giovanni

[baoshi]

Hi,

It is caused by the IRQ service times, if you need to operate at very high speeds then try the UART driver, it uses DMA.

Giovanni

Yes understand IRQ causes the delay. But why serving TXE takes longer time for higher baudrate? Should they take same time?

[Giovanni]

Hi,

It could be because other IRQ sources, is the UART IRQ at highest priority? usually it is not and that can cause jitter.

Giovanni