PIC18F2331/2431/4331/4431
Once the TXREG register transfers the data to the TSR
20.3 EUSART Asynchronous Mode
register (occurs in one TCY), the TXREG register is
empty and flag bit, TXIF (PIR1<4>), is set. This inter-
rupt can be enabled/disabled by setting/clearing
enable bit, TXIE (PIE1<4>). Flag bit, TXIF, will be set,
regardless of the state of enable bit TXIE and cannot be
cleared in software. Flag bit, TXIF, is not cleared
immediately upon loading the Transmit Buffer register,
TXREG. TXIF becomes valid in the second instruction
cycle following the load instruction. Polling TXIF
immediately following a load of TXREG will return
invalid results.
The Asynchronous mode of operation is selected by
clearing the SYNC bit (TXSTA<4>). In this mode, the
EUSART uses standard Non-Return-to-Zero (NRZ) for-
mat (one Start bit, eight or nine data bits and one Stop
bit). The most common data format is 8 bits. An on-chip
dedicated 8-bit/16-bit Baud Rate Generator can be
used to derive standard baud rate frequencies from the
oscillator.
The EUSART transmits and receives the LSb first. The
EUSART’s transmitter and receiver are functionally
independent, but use the same data format and baud
rate. The Baud Rate Generator produces a clock, either
x16 or x64 of the bit shift rate, depending on the BRGH
and BRG16 bits (TXSTA<2> and BAUDCON<3>). Par-
ity is not supported by the hardware but can be
implemented in software and stored as the 9th data bit.
While flag bit, TXIF, indicates the status of the TXREG
register, another bit, TRMT (TXSTA<1>), shows the
status of the TSR register. Status bit, TRMT, is a read-
only bit, which is set when the TSR register is empty.
No interrupt logic is tied to this bit, so the user has to
poll this bit in order to determine if the TSR register is
empty.
Asynchronous mode is available in all Low-Power
modes; it is available in Sleep mode only when Auto-
Wake-up on Sync Break is enabled. When in PRI_IDLE
mode, no changes to the Baud Rate Generator values
are required; however, other Low-Power mode clocks
may operate at another frequency than the primary
clock. Therefore, the Baud Rate Generator values may
need to be adjusted.
Note 1: The TSR register is not mapped in data
memory, so it is not available to the user.
2: Flag bit, TXIF, is set when enable bit,
TXEN, is set.
To set up an Asynchronous Transmission:
1. Initialize the SPBRGH:SPBRG registers for the
appropriate baud rate. Set or clear the BRGH
and BRG16 bits, as required, to achieve the
desired baud rate.
When operating in Asynchronous mode, the EUSART
module consists of the following important elements:
• Baud Rate Generator
• Sampling Circuit
2. Enable the asynchronous serial port by clearing
bit, SYNC, and setting bit, SPEN.
• Asynchronous Transmitter
• Asynchronous Receiver
3. If interrupts are desired, set enable bit, TXIE.
• Auto-Wake-up on Sync Break Character
• 12-Bit Break Character Transmit
• Auto-Baud Rate Detection
4. If 9-bit transmission is desired, set transmit bit,
TX9. Can be used as address/data bit.
5. Enable the transmission by setting bit, TXEN,
which will also set bit, TXIF.
20.3.1
EUSART ASYNCHRONOUS
TRANSMITTER
6. If 9-bit transmission is selected, the ninth bit
should be loaded in bit, TX9D.
The EUSART transmitter block diagram is shown in
Figure 20-2. The heart of the transmitter is the Transmit
(Serial) Shift Register (TSR). The Shift register obtains
its data from the Read/Write Transmit Buffer register,
TXREG. The TXREG register is loaded with data in
software. The TSR register is not loaded until the Stop
bit has been transmitted from the previous load. As
soon as the Stop bit is transmitted, the TSR is loaded
with new data from the TXREG register (if available).
7. Load data to the TXREG register (starts
transmission).
If using interrupts, ensure that the GIE and PEIE bits in
the INTCON register (INTCON<7:6>) are set.
DS39616D-page 226
2010 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
