63.7.3.11 Receiver Timeout

The Receiver Timeout provides support in handling variable-length frames. This feature detects an idle condition on the RXD line. When a timeout is detected, the FLEX_US_CSR.TIMEOUT bit rises and can generate an interrupt, thus indicating to the driver an end of frame.

The timeout delay period (during which the receiver waits for a new character) is programmed in the TO field of the Receiver Timeout register (FLEX_US_RTOR). If the TO field is written to 0, the Receiver Timeout is disabled and no timeout is detected. The FLEX_US_CSR.TIMEOUT bit remains at 0. Otherwise, the receiver loads a 16-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, the FLEX_US_CSR.TIMEOUT bit rises. Then, the user can either:

• Stop the counter clock until a new character is received. This is performed by writing a ‘1’ to FLEX_US_CR.STTTO. In this case, the idle state on RXD before a new character is received does not provide a timeout. This prevents having to handle an interrupt before a character is received and enables waiting for the next idle state on RXD after a frame is received.
• Obtain an interrupt while no character is received. This is performed by writing a ‘1’ to FLEX_US_CR.RETTO. In this case, the counter starts counting down immediately from the value TO. This generates a periodic interrupt so that a user timeout can be handled, for example when no key is pressed on a keyboard.

The following figure shows the block diagram of the Receiver Timeout feature.

The following table gives the maximum timeout period for some standard baud rates.