ATXMEGA128A4_1012 Datasheet, PDF (98/108 Pages) ATMEL Corporation – 8/16-bit XMEGA A4 Microcontroller

8069Q–AVR–12/10
XMEGA A4
23. Clearing TWI Stop Interrupt Flag may lock the bus
If software clears the STOP Interrupt Flag (APIF) on the same Peripheral Clock cycle as the
hardware sets this flag due to a new address received, CLKHOLD is not cleared and the
SCL line is not released. This will lock the bus.
Problem fix/Workaround
Check if the bus state is IDLE. If this is the case, it is safe to clear APIF. If the bus state is
not IDLE, wait for the SCL pin to be low before clearing APIF.
Code:
/* Only clear the interrupt flag if within a "safe zone". */
while ( /* Bus not IDLE: */
((COMMS_TWI.MASTER.STATUS & TWI_MASTER_BUSSTATE_gm) !=
TWI_MASTER_BUSSTATE_IDLE_gc)) &&
/* SCL not held by slave: */
!(COMMS_TWI.SLAVE.STATUS & TWI_SLAVE_CLKHOLD_bm)
)
{
/* Ensure that the SCL line is low */
if ( !(COMMS_PORT.IN & PIN1_bm) )
if ( !(COMMS_PORT.IN & PIN1_bm) )
break;
}
/* Check for an pending address match interrupt */
if ( !(COMMS_TWI.SLAVE.STATUS & TWI_SLAVE_CLKHOLD_bm) )
{
/* Safely clear interrupt flag */
COMMS_TWI.SLAVE.STATUS |= (uint8_t)TWI_SLAVE_APIF_bm;
}
24. TWI START condition at bus timeout will cause transaction to be dropped
If Bus Timeout is enabled and a timeout occurs on the same Peripheral Clock cycle as a
START is detected, the transaction will be dropped.
Problem fix/Workaround
None.
25. TWI Data Interrupt Flag erroneously read as set
When issuing the TWI slave response command CMD=0b11, it takes 1 Peripheral Clock
cycle to clear the data interrupt flag (DIF). A read of DIF directly after issuing the command
will show the DIF still set.
Problem fix/Workaround
Add one NOP instruction before checking DIF.
26. WDR instruction inside closed window will not issue reset
When a WDR instruction is execute within one ULP clock cycle after updating the window
control register, the counter can be cleared without giving a system reset.
Problem fix/Workaround
Wait at least one ULP clock cycle before executing a WDR instruction.
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