Datasheet

USART Operation: UART Mode www.ti.com
URXEIE is used to enable or disable erroneous characters from setting URXIFGx. When using
multiprocessor addressing modes, URXWIE is used to auto-detect valid address characters and reject
unwanted data characters.
Two types of characters do not set URXIFGx:
• Erroneous characters when URXEIE = 0
• Non-address characters when URXWIE = 1
When URXEIE = 1 a break condition sets the BRK bit and the URXIFGx flag.
18.2.7.3 Receive-Start Edge Detect Operation
The URXSE bit enables the receive start-edge detection feature. The recommended usage of the receive-
start edge feature is when BRCLK is sourced by the DCO and when the DCO is off because of low-power
mode operation. The ultra-fast turn-on of the DCO allows character reception after the start edge
detection.
When URXSE, URXIEx and GIE are set and a start edge occurs on URXDx, the internal signal URXS is
set. When URXS is set, a receive interrupt request is generated but URXIFGx is not set. User software in
the receive interrupt service routine can test URXIFGx to determine the source of the interrupt. When
URXIFGx = 0 a start edge was detected, and when URXIFGx = 1 a valid character (or break) was
received.
When the ISR determines the interrupt request was from a start edge, user software toggles URXSE, and
must enable the BRCLK source by returning from the ISR to active mode or to a low-power mode where
the source is active. If the ISR returns to a low-power mode where the BRCLK source is inactive, the
character is not received. Toggling URXSE clears the URXS signal and re-enables the start edge detect
feature for future characters. See chapter System Resets, Interrupts, and Operating Modes for information
on entering and exiting low-power modes.
The now active BRCLK allows the USART to receive the balance of the character. After the full character
is received and moved to UxRXBUF, URXIFGx is set and an interrupt service is again requested. Upon
ISR entry, URXIFGx = 1 indicating a character was received. The URXIFGx flag is cleared when user
software reads UxRXBUF.
; Interrupt handler for start condition and
; Character receive. BRCLK = DCO.
U0RX_Int BIT.B #URXIFG0,&IFG1 ; Test URXIFGx to determine
JZ ST_COND ; If start or character
MOV.B &UxRXBUF,dst ; Read buffer
... ;
RETI ;
ST_COND BIC.B #URXSE,&U0TCTL ; Clear URXS signal
BIS.B #URXSE,&U0TCTL ; Re-enable edge detect
BIC #SCG0+SCG1,0(SP) ; Enable BRCLK = DCO
RETI ;
NOTE: Break Detect With Halted UART Clock
When using the receive start-edge detect feature, a break condition cannot be detected
when the BRCLK source is off.
488
USART Peripheral Interface, UART Mode SLAU144J–December 2004–Revised July 2013
Submit Documentation Feedback
Copyright © 2004–2013, Texas Instruments Incorporated