Universal synchronous asynchronous receiver transmitter (USART) RM0008
799/1128 DocID13902 Rev 15
has been written). This means that it is not possible to receive a synchronous data without
transmitting data.
The LBCL, CPOL and CPHA bits have to be selected when both the transmitter and the
receiver are disabled (TE=RE=0) to ensure that the clock pulses function correctly. These
bits should not be changed while the transmitter or the receiver is enabled.
It is advised that TE and RE are set in the same instruction in order to minimize the setup
and the hold time of the receiver.
The USART supports master mode only: it cannot receive or send data related to an input
clock (CK is always an output).
Figure 288. USART example of synchronous transmission
Figure 289. USART data clock timing diagram (M=0)
RX
TX
CK
USART
Data out
Data in
Synchronous device
Clock
(e.g. slave SPI)
M=0 (8 data bits)
Clock (CPOL=0, CPHA=1)
Clock (CPOL=1, CPHA=0)
Clock (CPOL=1, CPHA=1)
Start LSB
MSB Stop
* LBCL bit controls last data clock pulse
Start
Idle or preceding
transmission
Data on TX
Stop
Clock (CPOL=0, CPHA=0)
01 234 56 7
*
*
*
*
Idle or next
transmission
*
Capture Strobe
LSB
MSB
Data on RX
01 234 56 7
(from master)
(from slave)
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