47.5.2.8 Receiver Time-out

The Receiver Time-out provides support in handling variable-length frames. This feature detects an idle condition on the URXD line. When a time-out is detected, UART_SR.TIMEOUT rises and can generate an interrupt, thus indicating to the driver an end of frame.

The time-out delay period (during which the receiver waits for a new character) is programmed in the field TO of the Receiver Time-out register (UART_RTOR). If TO is written to 0, the Receiver Time-out is disabled and no time-out is detected. UART_SR.TIMEOUT remains at ‘0’. Otherwise, the receiver loads an 8-bit counter with the value programmed in TO. This counter is decremented at each bit period and reloaded each time a new character is received. If the counter reaches 0, UART_SR.TIMEOUT rises. Then, the user can either:

• stop the counter clock until a new character is received. This is performed by writing a ‘1’ to UART_CR.STTTO. In this case, the idle state on URXD before a new character is received does not provide a time-out. This prevents having to handle an interrupt before a character is received and allows waiting for the next idle state on URXD after a frame is received, or
• obtain an interrupt while no character is received. This is performed by writing a ‘1’ to UART_CR.RETTO. If RETTO is performed, the counter starts counting down immediately from the TO value. This enables generation of a periodic interrupt so that a user time-out can be handled, for example when no key is pressed on a keyboard.

The figure below shows the block diagram of the Receiver Time-out feature.

The table below gives the maximum time-out period for some standard baud rates.