AT32UC3B0512_1 Datasheet, PDF (656/676 Pages) ATMEL Corporation – 32-bit AVR® Microcontroller

AT32UC3B
31.2.1.12 USART
3. Exceptions when system stack is protected by MPU
RETS behaves incorrectly when MPU is enabled and MPU is configured so thatsystem
stack is not readable in unprivileged mode.
Fix/Workaround
Workaround 1: Make system stack readable in unprivileged mode,
or
Workaround 2: Return from supervisor mode using rete instead of rets. This requires: 1.
Changing the mode bits from 001b to 110b before issuing the instruction.
Updating the mode bits to the desired value must be done using a single mtsr instruction so
it is done atomically. Even if this step is described in general as not safe in the UC technical
reference guide, it is safe in this very specific case.
2. Execute the RETE instruction.
4. Privilege violation when using interrupts in application mode with protected system
stack
If the system stack is protected by the MPU and an interrupt occurs in application mode, an
MPU DTLB exception will occur.
Fix/Workaround
Make a DTLB Protection (Write) exception handler which permits the interrupt request to be
handled in privileged mode.
1. ISO7816 info register US_NER cannot be read
The NER register always returns zero.
Fix/Workaround
None.
2. ISO7816 Mode T1: RX impossible after any TX
RX impossible after any TX.
Fix/Workaround
SOFT_RESET on RX+ Config US_MR + Config_US_CR
3. The RTS output does not function correctly in hardware handshaking mode
The RTS signal is not generated properly when the USART receives data in hardware hand-
shaking mode. When the Peripheral DMA receive buffer becomes full, the RTS output
should go high, but it will stay low.
Fix/Workaround
Do not use the hardware handshaking mode of the USART. If it is necessary to drive the
RTS output high when the Peripheral DMA receive buffer becomes full, use the normal
mode of the USART. Configure the Peripheral DMA Controller to signal an interrupt when
the receive buffer is full. In the interrupt handler code, write a one to the RTSDIS bit in the
USART Control Register (CR). This will drive the RTS output high. After the next DMA trans-
fer is started and a receive buffer is available, write a one to the RTSEN bit in the USART
CR so that RTS will be driven low.
4. Corruption after receiving too many bits in SPI slave mode
If the USART is in SPI slave mode and receives too much data bits (ex: 9bitsinstead of 8
bits) by the the SPI master, an error occurs . After that, the next reception may be corrupted
even if the frame is correct and the USART has been disabled, reseted by a soft reset and
re-enabled.
Fix/Workaround
None.
32059I–06/2010
656