PIC18F2420/2520/4420/4520
Once the TXREG register transfers the data to the TSR
18.2 EUSART Asynchronous Mode
register (occurs in one TCY), the TXREG register is empty
and the TXIF flag bit (PIR1<4>) is set. This interrupt can
be enabled or disabled by setting or clearing the interrupt
enable bit, TXIE (PIE1<4>). TXIF will be set regardless of
the state of TXIE; it cannot be cleared in software. TXIF
is also not cleared immediately upon loading TXREG, but
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>). Parity
is not supported by the hardware but can be
implemented in software and stored as the 9th data bit.
While TXIF indicates the status of the TXREG register,
another bit, TRMT (TXSTA<1>), shows the status of
the TSR register. 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.
Note 1: The TSR register is not mapped in data
When operating in Asynchronous mode, the EUSART
module consists of the following important elements:
memory so it is not available to the user.
2: Flag bit, TXIF, is set when enable bit,
• Baud Rate Generator
TXEN, is set.
• Sampling Circuit
To set up an Asynchronous Transmission:
• Asynchronous Transmitter
• Asynchronous Receiver
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.
• Auto-Wake-up on Sync Break Character
• 12-Bit Break Character Transmit
• Auto-Baud Rate Detection
2. Enable the asynchronous serial port by clearing
bit, SYNC, and setting bit, SPEN.
18.2.1
EUSART ASYNCHRONOUS
TRANSMITTER
3. If interrupts are desired, set enable bit, TXIE.
4. If 9-bit transmission is desired, set transmit bit,
TX9. Can be used as address/data bit.
The EUSART transmitter block diagram is shown in
Figure 18-3. 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).
5. Enable the transmission by setting bit, TXEN,
which will also set bit, TXIF.
6. If 9-bit transmission is selected, the ninth bit
should be loaded in bit, TX9D.
7. Load data to the TXREG register (starts
transmission).
8. If using interrupts, ensure that the GIE and PEIE
bits in the INTCON register (INTCON<7:6>) are
set.
FIGURE 18-3:
EUSART TRANSMIT BLOCK DIAGRAM
Data Bus
TXREG Register
TXIF
TXIE
8
MSb
(8)
LSb
Pin Buffer
and Control
TX pin
0
•
•
•
TSR Register
Interrupt
Baud Rate CLK
SPBRG
TXEN
TRMT
SPEN
BRG16
SPBRGH
TX9
Baud Rate Generator
TX9D
© 2008 Microchip Technology Inc.
DS39631E-page 211
MICROCHIP [ MICROCHIP ]
