Rev. 1.2
223
C8051T620/1/6/7 & C8051T320/1/2/3
26. UART1
UART1 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 “26.1. Baud Rate Generator” on page 223). A received data FIFO
allows UART1 to receive up to three data bytes before data is lost and an overflow occurs.
UART1 has six associated SFRs. Three are used for the Baud Rate Generator (SBCON1, SBRLH1, and
SBRLL1), two are used for data formatting, control, and status functions (SCON1, SMOD1), and one is
used to send and receive data (SBUF1). The single SBUF1 location provides access to both the transmit
holding register and the receive FIFO.
Writes to SBUF1 always access the Transmit Holding Register.
Reads of SBUF1 always access the first byte of the Receive FIFO; it is not possible to read data
from the Transmit Holding Register.
With UART1 interrupts enabled, an interrupt is generated each time a transmit is completed (TI1 is set in
SCON1), or a data byte has been received (RI1 is set in SCON1). The UART1 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 UART1 interrupt (transmit complete or receive
complete). Note that if additional bytes are available in the Receive FIFO, the RI1 bit cannot be cleared by
software.
Figure 26.1. UART1 Block Diagram
26.1. Baud Rate Generator
The UART1 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 SYSCLK frequencies.
The baud rate generator is configured using three registers: SBCON1, SBRLH1, and SBRLL1. The
UART1 Baud Rate Generator Control Register (SBCON1, SFR Definition ) enables or disables the baud
rate generator, and selects the prescaler value for the timer. The baud rate generator must be enabled for
UART1 to function. Registers SBRLH1 and SBRLL1 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. For reliable UART operation, it is recommended that the UART baud rate
is not configured for baud rates faster than SYSCLK/16. The baud rate for UART1 is defined in
Equation 26.1.
SBUF1
TX Holding
Register
RX FIFO
(3 Deep)
TX
Logic
RX
Logic
Write to SBUF1
Read of SBUF1
TX1
RX1
SMOD1
MC
E1
S1
PT1
S1
PT0
PE
1
S1
DL1
S1
DL0
XB
E1
SB
L1
Data Formatting
SCON1
OV
R
1
PE
RR
1
THR
E
1
RE
N
1
TB
X1
RBX
1
TI
1
RI
1
Control / Status
UART1
Interrupt
Timer (16-bit)
Pre-Scaler
(1, 4, 12, 48)
SYSCLK
SBRLH1
SBRLL1
Overflow
SBCON1
SB
1
R
U
N
SB
1P
S1
SB
1P
S0
EN
Baud Rate Generator