Figure 98. Rxne Behavior In Receive-Only Mode (Bdm = 0 And Rxonly = 1) - ST STM8S Reference Manual

RM0016

Figure 98. RXNE behavior in receive-only mode (BDM = 0 and RXONLY = 1).

Case of continuous transfers
Example with CPOL=1, CPHA=1, RXONLY=1
SCK
MISO/MOSI (in)
RXNE flag
Rx Buffer
(read SPI_DR)
Bidirectional receive procedure (BDM = 1 and BDOE = 0)
In this mode, the procedure is similar to the Receive-only procedure except that the BDM bit
must be set and the BDOE bit must be reset in the SPI_CR2 register before enabling the
SPI.
Continuous and discontinuous transfers
When transmitting data in master mode, if the software is fast enough to detect each TXE
rising edge (or TXE interrupt) and to immediately write the SPI_DR register before the
ongoing data transfer is complete, the communication is said to be continuous. In this case,
there is no discontinuity in the generation of the SPI clock between each data and the BSY
bit will never be reset between each data transfer.
On the contrary, if the software is not fast enough, this can lead to some discontinuities in
the communication. In this case, the BSY bit is reset between each data transmission (see
Figure 99).
In master receive-only mode (BDM = 0 and RXONLY = 1) or in bidirectional receive mode
(BDM = 1 and BDOE = 0), the communication is always continuous and the BSY flag is
always read at 1.
In slave mode, the continuity of the communication is decided by the SPI master device. But
even if the communication is continuous, the BSY flag goes low between each transfer for a
minimum duration of one SPI clock cycle (see
DATA 1 = 0xA1
b0 b1 b2 b3 b4 b5 b6 b7 b0 b1 b2 b3 b4 b5 b6 b7 b0 b1 b2 b3 b4 b5 b6 b7
set by hw
software waits until RXNE=1
and reads 0xA1 from SPI_DR and reads 0xA2 from SPI_DR
Doc ID 14587 Rev 8
Serial peripheral interface (SPI)
DATA 2 = 0xA2
cleared by sw
0xA1 0xA2
software waits until RXNE=1 software waits until RXNE=1
and reads 0xA3 from SPI_DR
Figure 95).
DATA 3 = 0xA3
0xA3
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