C8051F50x-F51x
24. UART0
UART0 is an asynchronous, full duplex serial port offering a variety of data formatting options. A dedicated
baud rate generator with a 16-bit timer and selectable prescaler is included, which can generate a wide
range of baud rates (details in Section “24.1. Baud Rate Generator” on page 243). A received data FIFO
allows UART0 to receive up to three data bytes before data is lost and an overflow occurs.
UART0 has six associated SFRs. Three are used for the Baud Rate Generator (SBCON0, SBRLH0, and
SBRLL0), two are used for data formatting, control, and status functions (SCON0, SMOD0), and one is
used to send and receive data (SBUF0). The single SBUF0 location provides access to both transmit and
receive registers. Writes to SBUF0 always access the Transmit register. Reads of SBUF0 always
access the buffered Receive register; it is not possible to read data from the Transmit register.
With UART0 interrupts enabled, an interrupt is generated each time a transmit is completed (TI0 is set in
SCON0), or a data byte has been received (RI0 is set in SCON0). The UART0 interrupt flags are not
cleared by hardware when the CPU vectors to the interrupt service routine. They must be cleared manually
by software, allowing software to determine the cause of the UART0 interrupt (transmit complete or receive
complete). If additional bytes are available in the Receive FIFO, the RI0 bit cannot be cleared by software.
TX
Logic
Data Formatting
TX0
Baud Rate Generator
SMOD0
SBRLH0 SBRLL0
Overflow
TX Holding
Register
Pre-Scaler
(1, 4, 12, 48)
SYSCLK
Timer (16-bit)
EN
Write to SBUF0
Read of SBUF0
SBUF0
Control / Status
SCON0
RX FIFO
(3 Deep)
SBCON0
RX
RX0
Logic
UART0
Interrupt
Figure 24.1. UART0 Block Diagram
24.1. Baud Rate Generator
The UART0 baud rate is generated by a dedicated 16-bit timer which runs from the controller’s core clock
(SYSCLK) and has prescaler options of 1, 4, 12, or 48. The timer and prescaler options combined allow for
a wide selection of baud rates over many clock frequencies.
The baud rate generator is configured using three registers: SBCON0, SBRLH0, and SBRLL0. The
UART0 Baud Rate Generator Control Register (SBCON0, SFR Definition 24.4) enables or disables the
baud rate generator, selects the clock source for the baud rate generator, and selects the prescaler value
for the timer. The baud rate generator must be enabled for UART0 to function. Registers SBRLH0 and
SBRLL0 contain a 16-bit reload value for the dedicated 16-bit timer. The internal timer counts up from the
reload value on every clock tick. On timer overflows (0xFFFF to 0x0000), the timer is reloaded. The baud
rate for UART0 is defined in Equation 24.1, where “BRG Clock” is the baud rate generator’s selected clock
source. For reliable UART operation, it is recommended that the UART baud rate is not configured for
baud rates faster than SYSCLK/16.
Rev. 1.1
243
SILICON [ SILICON ]
