Microchip Technology MA160014 Data Sheet
2010-2012 Microchip Technology Inc.
DS41412F-page 269
PIC18(L)F2X/4XK22
16.1
EUSART Asynchronous Mode
The EUSART transmits and receives data using the
standard non-return-to-zero (NRZ) format. NRZ is
implemented with two levels: a V
standard non-return-to-zero (NRZ) format. NRZ is
implemented with two levels: a V
OH
mark state which
represents a ‘1’ data bit, and a V
OL
space state which
represents a ‘0’ data bit. NRZ refers to the fact that
consecutively transmitted data bits of the same value
stay at the output level of that bit without returning to a
neutral level between each bit transmission. An NRZ
transmission port idles in the mark state. Each character
transmission consists of one Start bit followed by eight
or nine data bits and is always terminated by one or
more Stop bits. The Start bit is always a space and the
Stop bits are always marks. The most common data
format is eight bits. Each transmitted bit persists for a
period of 1/(Baud Rate). An on-chip dedicated 8-bit/16-
bit Baud Rate Generator is used to derive standard
baud rate frequencies from the system oscillator. See
consecutively transmitted data bits of the same value
stay at the output level of that bit without returning to a
neutral level between each bit transmission. An NRZ
transmission port idles in the mark state. Each character
transmission consists of one Start bit followed by eight
or nine data bits and is always terminated by one or
more Stop bits. The Start bit is always a space and the
Stop bits are always marks. The most common data
format is eight bits. Each transmitted bit persists for a
period of 1/(Baud Rate). An on-chip dedicated 8-bit/16-
bit Baud Rate Generator is used to derive standard
baud rate frequencies from the system oscillator. See
for examples of baud rate configurations.
The EUSART transmits and receives the LSb first. The
EUSART’s transmitter and receiver are functionally
independent, but share the same data format and baud
rate. Parity is not supported by the hardware, but can
be implemented in software and stored as the ninth
data bit.
EUSART’s transmitter and receiver are functionally
independent, but share the same data format and baud
rate. Parity is not supported by the hardware, but can
be implemented in software and stored as the ninth
data bit.
16.1.1
EUSART ASYNCHRONOUS
TRANSMITTER
TRANSMITTER
The EUSART transmitter block diagram is shown in
. The heart of the transmitter is the serial
Transmit Shift Register (TSR), which is not directly
accessible by software. The TSR obtains its data from
the transmit buffer, which is the TXREGx register.
accessible by software. The TSR obtains its data from
the transmit buffer, which is the TXREGx register.
16.1.1.1
Enabling the Transmitter
The EUSART transmitter is enabled for asynchronous
operations by configuring the following three control
bits:
operations by configuring the following three control
bits:
• TXEN = 1
• SYNC = 0
• SPEN = 1
All other EUSART control bits are assumed to be in
their default state.
their default state.
Setting the TXEN bit of the TXSTAx register enables the
transmitter circuitry of the EUSART. Clearing the SYNC
bit of the TXSTAx register configures the EUSART for
asynchronous operation. Setting the SPEN bit of the
RCSTAx register enables the EUSART and
automatically configures the TXx/CKx I/O pin as an
output. If the TXx/CKx pin is shared with an analog
peripheral the analog I/O function must be disabled by
clearing the corresponding ANSEL bit.
transmitter circuitry of the EUSART. Clearing the SYNC
bit of the TXSTAx register configures the EUSART for
asynchronous operation. Setting the SPEN bit of the
RCSTAx register enables the EUSART and
automatically configures the TXx/CKx I/O pin as an
output. If the TXx/CKx pin is shared with an analog
peripheral the analog I/O function must be disabled by
clearing the corresponding ANSEL bit.
16.1.1.2
Transmitting Data
A transmission is initiated by writing a character to the
TXREGx register. If this is the first character, or the
previous character has been completely flushed from
the TSR, the data in the TXREGx is immediately
transferred to the TSR register. If the TSR still contains
all or part of a previous character, the new character
data is held in the TXREGx until the Stop bit of the
previous character has been transmitted. The pending
character in the TXREGx is then transferred to the TSR
in one T
TXREGx register. If this is the first character, or the
previous character has been completely flushed from
the TSR, the data in the TXREGx is immediately
transferred to the TSR register. If the TSR still contains
all or part of a previous character, the new character
data is held in the TXREGx until the Stop bit of the
previous character has been transmitted. The pending
character in the TXREGx is then transferred to the TSR
in one T
CY
immediately following the Stop bit
transmission. The transmission of the Start bit, data bits
and Stop bit sequence commences immediately
following the transfer of the data to the TSR from the
TXREGx.
and Stop bit sequence commences immediately
following the transfer of the data to the TSR from the
TXREGx.
16.1.1.3
Transmit Data Polarity
The polarity of the transmit data can be controlled with
the CKTXP bit of the BAUDCONx register. The default
state of this bit is ‘0’ which selects high true transmit
idle and data bits. Setting the CKTXP bit to ‘1’ will invert
the transmit data resulting in low true idle and data bits.
The CKTXP bit controls transmit data polarity only in
Asynchronous mode. In Synchronous mode the
CKTXP bit has a different function.
the CKTXP bit of the BAUDCONx register. The default
state of this bit is ‘0’ which selects high true transmit
idle and data bits. Setting the CKTXP bit to ‘1’ will invert
the transmit data resulting in low true idle and data bits.
The CKTXP bit controls transmit data polarity only in
Asynchronous mode. In Synchronous mode the
CKTXP bit has a different function.
16.1.1.4
Transmit Interrupt Flag
The TXxIF interrupt flag bit of the PIR1/PIR3 register is
set whenever the EUSART transmitter is enabled and
no character is being held for transmission in the
TXREGx. In other words, the TXxIF bit is only clear
when the TSR is busy with a character and a new
character has been queued for transmission in the
TXREGx. The TXxIF flag bit is not cleared immediately
upon writing TXREGx. TXxIF becomes valid in the
second instruction cycle following the write execution.
Polling TXxIF immediately following the TXREGx write
will return invalid results. The TXxIF bit is read-only, it
cannot be set or cleared by software.
set whenever the EUSART transmitter is enabled and
no character is being held for transmission in the
TXREGx. In other words, the TXxIF bit is only clear
when the TSR is busy with a character and a new
character has been queued for transmission in the
TXREGx. The TXxIF flag bit is not cleared immediately
upon writing TXREGx. TXxIF becomes valid in the
second instruction cycle following the write execution.
Polling TXxIF immediately following the TXREGx write
will return invalid results. The TXxIF bit is read-only, it
cannot be set or cleared by software.
The TXxIF interrupt can be enabled by setting the
TXxIE interrupt enable bit of the PIE1/PIE3 register.
However, the TXxIF flag bit will be set whenever the
TXREGx is empty, regardless of the state of TXxIE
enable bit.
TXxIE interrupt enable bit of the PIE1/PIE3 register.
However, the TXxIF flag bit will be set whenever the
TXREGx is empty, regardless of the state of TXxIE
enable bit.
To use interrupts when transmitting data, set the TXxIE
bit only when there is more data to send. Clear the
TXxIE interrupt enable bit upon writing the last
character of the transmission to the TXREGx.
bit only when there is more data to send. Clear the
TXxIE interrupt enable bit upon writing the last
character of the transmission to the TXREGx.
Note:
The TXxIF transmitter interrupt flag is set
when the TXEN enable bit is set.
when the TXEN enable bit is set.