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USART Operation: UART Mode
13-6
USART Peripheral Interface, UART Mode
The URXWIE bit is used to control data reception in the idle-line
multiprocessor format. When the URXWIE bit is set, all non-address
characters are assembled but not transferred into the UxRXBUF, and
interrupts are not generated. When an address character is received, the
receiver is temporarily activated to transfer the character to UxRXBUF and
sets the URXIFGx interrupt flag. Any applicable error flag is also set. The user
can then validate the received address.
If an address is received, user software can validate the address and must
reset URXWIE to continue receiving data. If URXWIE remains set, only
address characters will be received. The URXWIE bit is not modified by the
USART hardware automatically.
For address transmission in idle-line multiprocessor format, a precise idle
period can be generated by the USART to generate address character
identifiers on UTXDx. The wake-up temporary (WUT) flag is an internal flag
double-buffered with the user-accessible TXWAKE bit. When the transmitter
is loaded from UxTXBUF, WUT is also loaded from TXWAKE resetting the
TXWAKE bit.
The following procedure sends out an idle frame to indicate an address
character will follow:
1) Set TXWAKE, then write any character to UxTXBUF. UxTXBUF must be
ready for new data (UTXIFGx = 1).
The TXWAKE value is shifted to WUT and the contents of UxTXBUF are
shifted to the transmit shift register when the shift register is ready for new
data. This sets WUT, which suppresses the start, data, and parity bits of a
normal transmission, then transmits an idle period of exactly 11 bits. When
two stop bits are used for the idle line, the second stop bit is counted as the
first mark bit of the idle period. TXWAKE is reset automatically.
2) Write desired address character to UxTXBUF. UxTXBUF must be ready
for new data (UTXIFGx = 1).
The new character representing the specified address is shifted out
following the address-identifying idle period on UTXDx. Writing the first
“don’t care” character to UxTXBUF is necessary in order to shift the
TXWAKE bit to WUT and generate an idle-line condition. This data is
discarded and does not appear on UTXDx.