Enabling THRE Interrupt :: Bugtracker

部分翻译

Enabling THRE Interrupt

Information:
Type :: BUG
Status :: CLOSED
Assigned to :: Howard, A. LeFevre

Description:
It is my understanding that an interrupt should be generated as soon as the Transmitter Holding Register Empty Interrupt is enabled (bit 1 IER). My simulation shows this is not the case.
Linux driver (8250.c) flags this behaviour with UART_BUG_TXEN, so maybe this is not a bug and this behaviour is intended.

Comments:

Santala, Jukka	Oct 11, 2007
The associated comment in 8250.c is actually: * Test for UARTs that do not reassert THRE when the * transmitter is idle and the interrupt has already * been cleared. Real 16550s should always reassert * this interrupt whenever the transmitter is idle and * the interrupt is enabled. Delays are necessary to * allow register changes to become visible. ...so it's a bug, but it doesn't kill serial port operation at least on Linux. However, it enables a 5Hz timeout timer per UART in Linux, which is a killer for low-power applications with multiple serial ports. 0.2 seconds is also a long time in some timing-critical applications (UPS signals, anyone?). Apparently, there are some compatibility-arguments for doing it either way, though, see http://www.ussg.iu.edu/hypermail/linux/kernel/0603.3/2543.html for example which seems to suggest a third, more complicated way of doing it. A quick web-search indicates something like that has been discussed on Opencores forums several years back as well. Unfortunately I've personally not had time to look into that in detail, so I don't have a solution to offer at this time, but add the above considerations to the issue.

Santala, Jukka

Oct 11, 2007

The associated comment in 8250.c is actually:
* Test for UARTs that do not reassert THRE when the
* transmitter is idle and the interrupt has already
* been cleared. Real 16550s should always reassert
* this interrupt whenever the transmitter is idle and
* the interrupt is enabled. Delays are necessary to
* allow register changes to become visible.
...so it's a bug, but it doesn't kill serial port operation at least on Linux. However, it enables a 5Hz timeout timer per UART in Linux, which is a killer for low-power applications with multiple serial ports. 0.2 seconds is also a long time in some timing-critical applications (UPS signals, anyone?). Apparently, there are some compatibility-arguments for doing it either way, though, see http://www.ussg.iu.edu/hypermail/linux/kernel/0603.3/2543.html for example which seems to suggest a third, more complicated way of doing it. A quick web-search indicates something like that has been discussed on Opencores forums several years back as well. Unfortunately I've personally not had time to look into that in detail, so I don't have a solution to offer at this time, but add the above considerations to the issue.

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Enabling THRE Interrupt