Datasheet

 2010 Microchip Technology Inc. DS39770C-page 235
PIC18F85J90 FAMILY
18.0 ENHANCED UNIVERSAL
SYNCHRONOUS
ASYNCHRONOUS RECEIVER
TRANSMITTER (EUSART)
PIC18F85J90 family devices have three serial I/O mod-
ules: the MSSP module, discussed in the previous
chapter and two Universal Synchronous Asynchronous
Receiver Transmitter (USART) modules. (Generically,
the USART is also known as a Serial Communications
Interface or SCI.) The USART can be configured as a
full-duplex, asynchronous system that can communi-
cate with peripheral devices, such as CRT terminals
and personal computers. It can also be configured as a
half-duplex synchronous system that can communicate
with peripheral devices, such as A/D or D/A integrated
circuits, serial EEPROMs, etc.
There are two distinct implementations of the USART
module in these devices: the Enhanced USART
(EUSART) discussed here and the Addressable
USART discussed in the next chapter. For this device
family, USART1 always refers to the EUSART, while
USART2 is always the AUSART.
The EUSART and AUSART modules implement the
same core features for serial communications; their
basic operation is essentially the same. The EUSART
module provides additional features, including Auto-
matic Baud Rate Detection and calibration, automatic
wake-up on Sync Break reception and 12-bit Break
character transmit. These features make it ideally
suited for use in Local Interconnect Network bus
(LIN/J2602 bus) systems.
The EUSART can be configured in the following
modes:
• Asynchronous (full-duplex) with:
- Auto-wake-up on character reception
- Auto-baud calibration
- 12-bit Break character transmission
• Synchronous – Master (half-duplex) with
selectable clock polarity
• Synchronous – Slave (half-duplex) with selectable
clock polarity
The pins of the EUSART are multiplexed with the
functions of PORTC (RC6/TX1/CK1/SEG27 and
RC7/RX1/DT1/SEG28). In order to configure these
pins as an EUSART:
• bit, SPEN (RCSTA1<7>), must be set (= 1)
• bit, TRISC<7>, must be set (= 1)
• bit, TRISC<6>, must be set (= 1)
The driver for the TX1 output pin can also be optionally
configured as an open-drain output. This feature allows
the voltage level on the pin to be pulled to a higher level
through an external pull-up resistor, and allows the out-
put to communicate with external circuits without the
need for additional level shifters.
The open-drain output option is controlled by the U1OD
bit (LATG<6>). Setting the bit configures the pin for
open-drain operation.
18.1 Control Registers
The operation of the Enhanced USART module is
controlled through three registers:
• Transmit Status and Control Register 1 (TXSTA1)
• Receive Status and Control Register 1 (RCSTA1)
• Baud Rate Control Register 1 (BAUDCON1)
The registers are described in Register 18-1,
Register 18-2 and Register 18-3.
Note: The EUSART control will automatically
reconfigure the pin from input to output as
needed.