Rev. 1.10
126
November 04, 2019
Rev. 1.10
127
November 04, 2019
HT45F5Q-3
Battery Charger Flash MCU
HT45F5Q-3
Battery Charger Flash MCU
UART Transmitter
Data word lengths of either 8 or 9 bits can be selected by programming the UBNO bit in the UUCR1
register. When UBNO bit is set, the word length will be set to 9 bits. In this case the 9th bit, which is
the MSB, needs to be stored in the UTX8 bit in the UUCR1 register. At the transmitter core lies the
Transmitter Shift Register, more commonly known as the TSR, whose data is obtained from the transmit
data register, which is known as the UTXR_RXR register. The data to be transmitted is loaded into
this UTXR_RXR register by the application program. The TSR register is not written to with new data
until the stop bit from the previous transmission has been sent out. As soon as this stop bit has been
transmitted, the TSR can then be loaded with new data from the UTXR_RXR register, if it is available.
It should be noted that the TSR register, unlike many other registers, is not directly mapped into the Data
Memory area and as such is not available to the application program for direct read/write operations.
An actual transmission of data will normally be enabled when the UTXEN bit is set, but the data will
not be transmitted until the UTXR_RXR register has been loaded with data and the baud rate generator
has defined a shift clock source. However, the transmission can also be initiated by first loading data
into the UTXR_RXR register, after which the UTXEN bit can be set. When a transmission of data
begins, the TSR is normally empty, in which case a transfer to the UTXR_RXR register will result in an
immediate transfer to the TSR. If during a transmission the UTXEN bit is cleared, the transmission will
immediately cease and the transmitter will be reset. The TX output pin can then be configured as the I/O
or other pin-shared function by configuring the corresponding pin-shared control bits.
Transmitting Data
When the UART is transmitting data, the data is shifted on the TX pin from the shift register, with
the least significant bit first. In the transmit mode, the UTXR_RXR register forms a buffer between
the internal bus and the transmitter shift register. It should be noted that if 9-bit data format has been
selected, then the MSB will be taken from the UTX8 bit in the UUCR1 register. The steps to initiate
a data transfer can be summarized as follows:
•
Make the correct selection of the UBNO, UPRT, UPREN and USTOPS bits to define the required
word length, parity type and number of stop bits.
•
Setup the UBRG register to select the desired baud rate.
•
Set the UTXEN bit to ensure that the TX pin is used as a UART transmitter pin.
•
Access the UUSR register and write the data that is to be transmitted into the UTXR_RXR
register. Note that this step will clear the UTXIF bit.
This sequence of events can now be repeated to send additional data.
It should be noted that when UTXIF=0, data will be inhibited from being written to the UTXR_RXR
register. Clearing the UTXIF flag is always achieved using the following software sequence:
1. A UUSR register access
2. A UTXR_RXR register write execution
The read-only UTXIF flag is set by the UART hardware and if set indicates that the UTXR_RXR
register is empty and that other data can now be written into the UTXR_RXR register without
overwriting the previous data. If the UTEIE bit is set then the UTXIF flag will generate an interrupt.
During a data transmission, a write instruction to the UTXR_RXR register will place the data
into the UTXR_RXR register, which will be copied to the shift register at the end of the present
transmission. When there is no data transmission in progress, a write instruction to the UTXR_RXR
register will place the data directly into the shift register, resulting in the commencement of data
transmission, and the UTXIF bit being immediately set. When a frame transmission is complete,
which happens after stop bits are sent or after the break frame, the UTIDLE bit will be set. To clear
the UTIDLE bit the following software sequence is used:
