SV: [ECOS] Ethernet traffic causing loss of bytes on UART

Alex Lindeijer
Tue Sep 1 16:30:00 GMT 2009

When we were missing bytes on the serial port and we pulled the ethernet cable out, we received all bytes on the serial port. First we tried increasing the serial port interrupt priority since it actually tolerates less interupt latency than the ethernet port.
I'll have to study the MPC8541 reference guide more later but as we understand it:
The interupt controller in the MPC8541 needs to have the interrupt acknowledge before it handles any new interrupts(like from the UART). So, in the ISR of the ethernet driver we mask the ethernet interrupt and acknowledge the interrupt which allows the interrupt controller to process new interupts /call ISRs. In the DSR of the ethernet driver we service the ethernet interrupt and unmask it when done.
If we acknowledge the interrupt before leaving the DSR, no other interrupt given by the interrupt controller whilst processing the ethernet frames. That is why we lost bytes on the serial port.
At least that is our theory, which I have to confirm by checking the documentation closer.
In pratice we are now receiving/sending traffic on both ethernet and serial port undisturbed. 
This also the we I have seen it done in other drivers (also an alternative tsec driver).
This is we think it works. But I appreciate any comments and tips. Interupt handling and driver software is certainly not my strongest point.


Fra: Gary Thomas []
Sendt: ti 01.09.2009 17:24
Til: Alex Lindeijer
Emne: Re: [ECOS] Ethernet traffic causing loss of bytes on UART

On 09/01/2009 08:36 AM, Alex Lindeijer wrote:
> Hi All
> This problem is fixed. The drivers that came with our Python MPC8541
> eval board, acknowledged the ethernet interrput in the DSR (after
> processing!) and not in the ISR.
> It seems to work fine now.

How does that have anything to do with losing bytes on the UART?

BTW, IMO it's not safe/proper to acknowledge the interrupt in the ISR.
The ISR masks the interrupt and only after the network stack has fully
processed the data is it acknowledged and unmasked.  Acking the interrupt
in the ISR could lead to missing some interrupts.

> -----Original Message-----
> From:
> [] On Behalf Of Alex
> Lindeijer
> Sent: 1. september 2009 08:24
> To:
> Subject: [ECOS] Ethernet traffic causing loss of bytes on UART
> Hi
> We are running a serial port on 38400 baud and are seeing loss of Rx
> bytes. When unplugging the ethernet cable we have no problems.
> We are using a MPC8541, it has a PC16550D programming model for the
> UART. I see that the driver uses the FIFO and reads until the FIFO is
> empty.
> The Ethernet driver is a tsec driver .
> We encountered similar problems when writing to flash. We got some
> verification/write errors with Ethernet traffic going. In that case we
> masked the interrupts while writing to flash. But we don't want to do
> that when receiving bytes on the serial port of course.
> Has anybody else encounter such problems? Any hints in were to look? We
> have been looking into this now for some time and are getting a bit
> desperate.....;-(
> Cheers
> Alex Lindeijer

Gary Thomas                 |  Consulting for the
MLB Associates              |    Embedded world

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