PIC18F47J53 Datasheet, PDF (355/586 Pages) Microchip Technology – 28/44-Pin, High-Performance USB Microcontrollers with nanoWatt XLP Technology

PIC18F47J53 FAMILY
21.2 EUSART Asynchronous Mode
The Asynchronous mode of operation is selected by
clearing the SYNC bit (TXSTAx<4>). In this mode, the
EUSART uses standard Non-Return-to-Zero (NRZ)
format (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 BRG 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 BRG produces a clock, either x16 or x64 of the
bit shift rate, depending on the BRGH and BRG16 bits
(TXSTAx<2> and BAUDCONx<3>). Parity is not
supported by the hardware but can be implemented in
software and stored as the ninth data bit.
When operating in Asynchronous mode, the EUSART
module consists of the following important elements:
• Baud Rate Generator
• Sampling Circuit
• Asynchronous Transmitter
• Asynchronous Receiver
• Auto-Wake-up on Sync Break Character
• 12-Bit Break Character Transmit
• Auto-Baud Rate Detection
21.2.1 EUSART ASYNCHRONOUS
TRANSMITTER
Figure 21-3 displays the EUSART transmitter block
diagram.
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, TXREGx. The
TXREGx 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 TXREGx register (if available).
Once the TXREGx register transfers the data to the TSR
register (occurs in one TCY), the TXREGx register is
empty and the TXxIF flag bit is set. This interrupt can be
enabled or disabled by setting or clearing the interrupt
enable bit, TXxIE. TXxIF will be set regardless of the
state of TXxIE; it cannot be cleared in software. TXxIF is
also not cleared immediately upon loading TXREGx, but
becomes valid in the second instruction cycle following
the load instruction. Polling TXxIF immediately following
a load of TXREGx will return invalid results.
While TXxIF indicates the status of the TXREGx
register; another bit, TRMT (TXSTAx<1>), shows the
status of the TSR register. TRMT is a read-only bit,
which is set when the TSR register is empty. No inter-
rupt 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
memory, so it is not available to the user.
2: Flag bit, TXxIF, is set when enable bit,
TXEN, is set.
To set up an Asynchronous Transmission:
1. Initialize the SPBRGHx:SPBRGx registers for
the appropriate baud rate. Set or clear the
BRGH and BRG16 bits, as required, to achieve
the desired baud rate.
2. Enable the asynchronous serial port by clearing
bit, SYNC, and setting bit, SPEN.
3. If interrupts are desired, set enable bit, TXxIE.
4. If 9-bit transmission is desired, set transmit bit,
TX9; can be used as an address/data bit.
5. Enable the transmission by setting bit, TXEN,
which will also set bit, TXxIF.
6. If 9-bit transmission is selected, the ninth bit
should be loaded in bit, TX9D.
7. Load data to the TXREGx register (starts
transmission).
8. If using interrupts, ensure that the GIE and PEIE
bits in the INTCON register (INTCON<7:6>) are
set.
FIGURE 21-3:
EUSART TRANSMIT BLOCK DIAGRAM
TXxIF
TXxIE
MSb
(8)
Interrupt
TXEN Baud Rate CLK
BRG16
SPBRGHx SPBRGx
Baud Rate Generator
Data Bus
TXREGx Register
8

TSR Register
TX9
TX9D
LSb
0
Pin Buffer
and Control
TRMT
SPEN
TXx Pin
 2010 Microchip Technology Inc.
Preliminary
DS39964B-page 355