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Standard 8051 8-Bit Flash MCU
HT85F2260/HT85F2270/HT85F2280
UART Serial Interfaces – UART0 and UART1
Basic UART Data Transfer Scheme
The block diagram shows the overall UART data transfer structure arrangement. For data
transmission, the actual data to be transmitted from the MCU is first transferred to the transmitted
register by the application program. The data will then be transferred to the Transmit Shift Register
from where it will be shifted out, LSB first, onto the transmitter pin, TXDn, at a rate controlled by
the Baud Rate Generator. Only the transmitted register is mapped onto the MCU Data Memory, the
Transmit Shift Register is not mapped and is therefore inaccessible to the application program.
For data reception, data to be received by the UART is accepted on the external receive pin,
RXDn, from where it is shifted in, LSB first, to the Receiver Shift Register at a rate controlled
by the Baud Rate Generator. When the shift register is full, the data will then be transferred from
the shift register to the internal received register, where it is buffered and can be manipulated by
the application program. Only the received register is mapped onto the MCU Data Memory, the
Receiver Shift Register is not mapped and is therefore inaccessible to the application program.
It should be noted that the actual register for data transmission and reception, only exists as a single
shared register in the Data Memory, known as the S0BUF register in the UART0 or the S1BUF
register in the UART1, and is used for both data transmission and data reception.
T r a n s m i t R e g i s t e r
R e c e i v e R e g i s t e r
M S B
L S B
T r a n s m i t t e r S h i f t R e g i s t e r
T r a n s m i t
T X D n P i n
M S B
L S B
R e c e i v e r S h i f t R e g i s t e r
R e c e i v e
R X D n P i n
B a u d R a t e
G e n e r a t o r
C L K
C L K
B u f f e r
M C U D a t a B u s
Basic UART Data Transfer Diagram