Rev. 1.50
156
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Rev. 1.50
15�
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HT66F0175/HT66F0185
A/D Flash MCU with EEPROM
HT66F0175/HT66F0185
A/D Flash MCU with EEPROM
UART External Pin
To communicate with an external serial interface, the internal UART has two external pins known
as TX and RX. The TX and RX pins are respectively the UART transmitter and receiver pins which
are pin-shared with I/O or other pin-shared functions. Along with the UARTEN bit, the TXEN and
RXEN bits, if set, will automatically setup these I/O pins to their respective TX output and RX input
conditions and disable any pull-high resistor option which may exist on the TX and RX pins. When
the TX or RX pin function is disabled by clearing the UARTEN, TXEN or RXEN bit, the TX or RX
pin will be used as I/O or other pin-shared functional pin depending upon the pin-shared function
priority.
UART Data Transfer Scheme
The above diagram shows the overall data transfer structure arrangement for the UART interface.
The actual data to be transmitted from the MCU is first transferred to the TXR 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 TX pin at a rate controlled by the Baud Rate Generator. Only
the TXR register is mapped onto the MCU Data Memory, the Transmit Shift Register is not mapped
and is therefore inaccessible to the application program.
Data to be received by the UART is accepted on the external RX pin, 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 RXR
register, where it is buffered and can be manipulated by the application program. Only the TXR
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, although referred to
in the text, and in application programs, as separate TXR and RXR registers, only exists as a single
shared register in the Data Memory. This shared register known as the TXR_RXR register is used
for both data transmission and data reception.
UART Status and Control Registers
There are five control registers associated with the UART function. The USR, UCR1 and UCR2
registers control the overall function of the UART, while the BRG register controls the Baud rate.
The actual data to be transmitted and received on the serial interface is managed through the TXR_
RXR data registers.
Register
Name
Bit
7
6
5
4
3
2
1
0
USR
PERR
NF
FERR
OERR
RIDLE
RXIF
TIDLE
TXIF
UCR1
U�RTEN
BNO
PREN
PRT
STOPS
TXBRK
RX�
TX�
UCR�
TXEN
RXEN
BRGH
�DDEN
W�KE
RIE
TIIE
TEIE
BRG
BRG�
BRG6
BRG5
BRG4
BRG3
BRG�
BRG1
BRG0
TXR_RXR TXRX�
TXRX6
TXRX5
TXRX4
TXRX3
TXRX�
TXRX1
TXRX0
UART Status and Control Registers List – HT66F0185 only