SPRUJ17H March 2022 – October 2024 AM2631 , AM2631-Q1 , AM2632 , AM2632-Q1 , AM2634 , AM2634-Q1
When the SCI receives data and transfers that data from SCIRXSHF to SCIRD, the RXRDY bit is set and if SETRXINT is set, the SCI also generates an interrupt. The interrupt triggers the CPU to read the newly received frame before another one is received. In multiprocessor communication modes, this default behavior can be enhanced to provide selective indication of new data. When the SCI receives an address frame that does not match the address, the device can ignore the data following this non-matching address until the next address frame by using sleep mode. Sleep mode can be used with both idle-line and address-bit multiprocessor modes.
If sleep mode is enabled by the SLEEP bit, then the SCI transfers data from SCIRXSHF to SCIRD only for address frames. Therefore, in sleep mode, all data frames are assembled in the SCIRXSHF register without being shifted into the SCIRD and without initiating a receive interrupt or DMA request. Upon reception of an address frame, the contents of the SCIRXSHF are moved into SCIRD, and the software must read SCIRD and determine if the SCI is being addressed by comparing the received address against the address previously set in the software and stored somewhere in memory (the SCI does not have hardware available for address comparison). If the SCI is being addressed, the software must clear the SLEEP bit so that the SCI loads SCIRD with the data of the data frames that follow the address frame.
When the SCI has been addressed and sleep mode has been disabled (in software) to allow the receipt of data, the SCI can check the RXWAKE bit (SCIFLR.12) to determine when the next address has been received. The bit is set to 1, if the current value in SCIRD is an address; the bit is set to 0, if SCIRD contains data. If the RXWAKE bit is set, then software can check the address in SCIRD against the address. If SCIRD is still being addressed, then sleep mode can remain disabled; otherwise, the SLEEP bit can be set again.
Following is a sequence of events typical of sleep mode operation:
By ignoring data frames that are not intended for the device, fewer interrupts are generated. Otherwise, these interrupts require CPU intervention to read data that is of no significance to this specific device. Using sleep mode can help free some CPU resources.
Except for the RXRDY flag, the SCI continues to update the receiver status flags (see Table 13-289) while sleep mode is active. In this way, if an error occurs on the receive line, an application can immediately respond to the error and take the appropriate corrective action.
Because the RXRDY bit is not updated for data frames when sleep mode is enabled, the SCI can enable sleep mode and use a polling algorithm if desired. In this case, when RXRDY is set, software knows that a new address has been received. If the SCI is not being addressed, then the software can not change the value of the SLEEP bit and can continue to poll RXRDY.