XMTG: This bit is set to indicate that the USART is transmit-
ting. It gets reset at the end of the last frame (end of last Stop
bit). Read only, cleared on reset.
14.0 USART (Continued)
XBIT9/PSEL0: Programs the ninth bit for transmission when
the USART is operating with nine data bits per frame. For
seven or eight data bits per frame, this bit in conjunction with
PSEL1 selects parity. Read/Write, cleared on reset.
RCVG: This bit is set high whenever a framing error or a
Break Detect occurs and goes low when RDX goes high.
Read only, cleared on reset.
CHL1, CHL0: These bits select the character frame format.
Parity is not included and is generated/verified by hardware.
Read/Write, cleared on reset.
ENUI—USART INTERRUPT AND CLOCK SOURCE REG-
ISTER (Address at 0BC)
STP2
Bit 7
BRK
ETDX
SSEL XRCLK XTCLK ERI
ETI
CHL1 = 0, CHL0 = 0
CHL1 = 0, CHL0 = 1
CHL1 = 1, CHL0 = 0
CHL1 = 1, CHL0 = 1
The frame contains eight data bits.
The frame contains seven data bits.
The frame contains nine data bits.
Bit 0
STP2: This bit programs the number of Stop bits to be
transmitted. Read/Write, cleared on reset.
Loopback Mode selected. Trans-
mitter output internally looped back
to receiver input. Nine bit framing
format is used.
STP2 = 0
STP2 = 1
One Stop bit transmitted.
Two Stop bits transmitted.
BRK: Holds TDX (USART Transmit Pin) low to generate a
Line Break. Timing of the Line Break is under software
control.
ERR: This bit is a global USART error flag which gets set if
any or a combination of the errors (DOE, FE, PE, BO) occur.
Read only; it cannot be written by software, cleared on reset.
ETDX: TDX (USART Transmit Pin) is the alternate function
assigned to Port L pin L2; it is selected by setting ETDX bit.
RBFL: This bit is set when the USART has received a
complete character and has copied it into the RBUF register.
It is automatically reset when software reads the character
from RBUF. Read only; it cannot be written by software,
cleared on reset.
SSEL: USART mode select. Read only, cleared on reset.
SSEL = 0
SSEL = 1
Asynchronous Mode.
Synchronous Mode.
TBMT: This bit is set when the USART transfers a byte of
data from the TBUF register into the TSFT register for trans-
mission. It is automatically reset when software writes into
the TBUF register. Read only, bit is set to “one” on reset; it
cannot be written by software.
XRCLK: This bit selects the clock source for the receiver
section. Read/Write, cleared on reset.
XRCLK = 0
The clock source is selected through the
PSR and BAUD registers.
XRCLK = 1
Signal on CKX (L1) pin is used as the clock.
ENUR—USART RECEIVE CONTROL AND STATUS REG-
ISTER (Address at 0BB)
XTCLK: This bit selects the clock source for the transmitter
section. Read/Write, cleared on reset.
DOE
Bit 7
FE
PE
BD
RBIT9 ATTN XMTG RCVG
Bit 0
XTCLK = 0
The clock source is selected through the PSR
and BAUD registers.
XTCLK = 1
Signal on CKX (L1) pin is used as the clock.
DOE: Flags a Data Overrun Error. Read only, cleared on
read, cleared on reset.
ERI: This bit enables/disables interrupt from the receiver
section. Read/Write, cleared on reset.
DOE = 0
Indicates no Data Overrun Error has been de-
tected since the last time the ENUR register
was read.
ERI = 0
ERI = 1
Interrupt from the receiver is disabled.
Interrupt from the receiver is enabled.
ETI: This bit enables/disables interrupt from the transmitter
section. Read/Write, cleared on reset.
DOE = 1
Indicates the occurrence of a Data Overrun
Error.
ETI = 0
ETI = 1
Interrupt from the transmitter is disabled.
Interrupt from the transmitter is enabled.
FE: Flags a Framing Error. Read only, cleared on read,
cleared on reset.
FE = 0
Indicates no Framing Error has been detected
since the last time the ENUR register was read.
14.3 ASSOCIATED I/O PINS
Data is transmitted on the TDX pin and received on the RDX
pin. TDX is the alternate function assigned to Port L pin L2;
it is selected by setting ETDX (in the ENUI register) to one.
RDX is an inherent function Port L pin L3, requiring no setup.
Port L pin L2 must be configured as an output in the Port L
Configuration Register in order to be used as the TDX pin.
FE = 1
Indicates the occurrence of a Framing Error.
PE: Flags a Parity Error. Read only, cleared on read, cleared
on reset.
PE = 0
Indicates no Parity Error has been detected since
the last time the ENUR register was read.
PE = 1
Indicates the occurrence of a Parity Error.
The baud rate clock for the USART can be generated on-
chip, or can be taken from an external source. Port L pin L1
(CKX) is the external clock I/O pin. The CKX pin can be
either an input or an output, as determined by Port L Con-
figuration and Data registers (Bit 1). As an input, it accepts a
clock signal which may be selected to drive the transmitter
and/or receiver. As an output, it presents the internal Baud
Rate Generator output.
BD: Flags a line break.
BD = 0 Indicates no Line Break has been detected since
the last time the ENUR register was read.
BD = 1 Indicates the occurrence of a Line Break.
RBIT9: Contains the ninth data bit received when the
USART is operating with nine data bits per frame. Read only,
cleared on reset.
Note: The CKX pin is unavailable if Port L1 is used for the
Low Speed Oscillator.
ATTN: ATTENTION Mode is enabled while this bit is set.
This bit is cleared automatically on receiving a character with
data bit nine set. Read/Write, cleared on reset.
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