Overrun errors with two USART interrupts

Question

Using two USARTs running at 115200 baud on a STM32F2, one to communicate with a radio module and one for serial from a PC. The clock speed is 120MHz.

When receiving data from both USARTs simultaneously overrun errors can occur on one USART or the other. Doing some quick back of the envelope calculations there should be enough time to process both, as the interrupts are just simple copy the byte to a circular buffer.

In both theory and from measurement the interrupt code to push byte to buffer should/does run in the order of 2-4µS, at 115200 baud we have around 70us to process each char.

Why are we seeing occassional OREs on one or other USART?

Update - additional information:

1. No other ISRs in our code are firing at this time.
2. We are running Keil RTX with systick interrupt configured to fire every 10mS.
3. We are not disabling any interrupts at this time.
4. According this book (The Designer's Guide to the Cortex-M Processor Family) the interupt latency is around 12cycles (not really deadly)

Given all the above 70us is at least a factor of 10 over the time we take to clear the interrupts - so I'm not sure its is so easy to explain. Should I be concluding that there must be some other factor I am over looking?

MDK-ARM is version 4.70

The systick interrupt is used by the RTOS so cannot time this the other ISRs take 2-3µS to run per byte each.

Solution

I ran into a similar problem as yours a few months ago on a Cortex M4 (SAM4S). I have a function that gets called at 100 Hz based on a Timer Interrupt.

In the meantime I had a UART configured to interrupt on char reception. The expected data over UART was 64 byte long packets and interrupting on every char caused latency such that my 100 Hz update function was running at about 20 Hz. 100 Hz is relatively slow on this particular 120 MHz processor but interrupting on every char was causing massive delays.

I decided to configure the UART to use PDC (Peripheral DMA controller) and my problems disappeared instantly.

DMA allows the UART to store data in memory WITHOUT interrupting the processor until the buffer is full saving lots of overhead.

In my case, I told PDC to store UART data into an buffer (byte array) and specified the length. When UART via PDC filled the buffer the PDC issued an interrupt.

In PDC ISR:

1. Give PDC new empty buffer
2. Restart UART PDC (so can collect data while we do other stuff in isr)
3. memcpy full buffer into RINGBUFFER
4. Exit ISR

As swineone recommended above, implement DMA and you'll love life.