COP888XG Datasheet, PDF (26/53 Pages) National Semiconductor (TI) – 8-Bit CMOS ROM Based Microcontrollers with 4k to 24k Memory, Comparators and USART

USART (Continued)
CHL1, CHL0: These bits select the character frame format.
Parity is not included and is generated/verified by hardware.
Read/Write, cleared on reset.
CHL1 = 0, CHL0 = 0 The frame contains eight data bits.
CHL1 = 0, CHL0 = 1 The frame contains seven data
bits.
CHL1 = 1, CHL0 = 0 The frame contains nine data bits.
CHL1 = 1, CHL0 = 1
Loopback Mode selected. Trans-
mitter output internally looped back
to receiver input. Nine bit framing
format is used.
ERR: This bit is a global USART error flag which gets set if
any or a combination of the errors (DOE, FE, PE) occur.
Read only; it cannot be written by software, cleared on reset.
RBFL: This bit is set when the USART has received a com-
plete 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.
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.
ENUR-USART Receive Control and Status Register
(Address at 0BB)
DOE FE PE Reserved RBIT9 ATTN XMTG RCVG
(Note 12)
Bit 7
Bit 0
Note 12: Bit is reserved for future use. User must set to zero.
DOE: Flags a Data Overrun Error. Read only, cleared on
read, cleared on reset.
DOE = 0
Indicates no Data Overrun Error has been de-
tected since the last time the ENUR register
was read.
DOE = 1 Indicates the occurrence of a Data Overrun Er-
ror.
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.
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.
SPARE: Reserved for future use. Read/Write, cleared on re-
set.
RBIT9: Contains the ninth data bit received when the US-
ART is operating with nine data bits per frame. Read only,
cleared on reset.
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.
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.
RCVG: This bit is set high whenever a framing error occurs
and goes low when RDX goes high. Read only, cleared on
reset.
ENUI-USART Interrupt and Clock Source Register
(Address at 0BC)
STP2 STP78 ETDX SSEL XRCLK XTCLK ERI ETI
Bit 7
Bit 0
STP2: This bit programs the number of Stop bits to be trans-
mitted. Read/Write, cleared on reset.
STP2 = 0 One Stop bit transmitted.
STP2 = 1 Two Stop bits transmitted.
STP78: This bit is set to program the last Stop bit to be 7/8th
of a bit in length. Read/Write, 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.
To simulate line break generation, software should reset
ETDX bit and output logic zero to TDX pin through Port L
data and configuration registers. Read/Write, cleared on re-
set.
SSEL: USART mode select. Read/Write, cleared on reset.
SSEL = 0 Asynchronous Mode.
SSEL = 1 Synchronous Mode.
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.
XTCLK: This bit selects the clock source for the transmitter
section. Read/Write, cleared on reset.
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.
ERI: This bit enables/disables interrupt from the receiver
section. Read/Write, cleared on reset.
ERI = 0 Interrupt from the receiver is disabled.
ERI = 1 Interrupt from the receiver is enabled.
ETI: This bit enables/disables interrupt from the transmitter
section. Read/Write, cleared on reset.
ETI = 0 Interrupt from the transmitter is disabled.
ETI = 1 Interrupt from the transmitter is enabled.
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 of Port L pin L3, requiring no
setup.
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.
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