Rev. 1.10
168
October 23, 2020
Rev. 1.10
169
October 23, 2020
BC66F5652
2.4GHz RF Transceiver A/D Flash MCU
BC66F5652
2.4GHz RF Transceiver A/D Flash MCU
The following diagram shows the transmit and receive waveforms for both 8-bit and 9-bit data formats.
Bit 0
8-bit data format
Bit 1
Stop
Bit
Next
Start
Bit
Start
Bit
Parity Bit
Bit 2
Bit 3
Bit 4
Bit 5
Bit 6
Bit 7
Bit 0
9-bit data format
Bit 1
Start
Bit
Bit 2
Bit 3
Bit 4
Bit 5
Bit 6
Stop
Bit
Next
Start
Bit
Parity Bit
Bit 8
Bit 7
UART Transmitter
Data word lengths of either 8 or 9 bits can be selected by programming the BNO bit in the UCR1
register. When BNO 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 TX8 bit in the UCR1 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 TXR_RXR register. The data to be transmitted
is loaded into this TXR_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 TXR_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
TXEN bit is set, but the data will not be transmitted until the TXR_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 TXR_RXR register, after which the TXEN bit can
be set. When a transmission of data begins, the TSR is normally empty, in which case a transfer to
the TXR_RXR register will result in an immediate transfer to the TSR. If during a transmission the
TXEN 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 TXR_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 TX8 bit in the UCR1 register. The steps to initiate a
data transfer can be summarized as follows:
•
Make the correct selection of the BNO, PRT, PREN and STOPS bits to define the required word
length, parity type and number of stop bits.
• Setup the BRG register to select the desired baud rate.
• Set the TXEN bit to ensure that the TX pin is used as a UART transmitter pin.
• Access the USR register and write the data that is to be transmitted into the TXR_RXR register.
Note that this step will clear the TXIF bit.
This sequence of events can now be repeated to send additional data.
It should be noted that when TXIF=0, data will be inhibited from being written to the TXR_RXR
register. Clearing the TXIF flag is always achieved using the following software sequence:
1. A USR register access
2. A TXR_RXR register write execution
