Chapter 1 Hardware Structure
mission of data begins, the TSR is normally empty, in which case a transfer to the TXR 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 will then re-
turn to having a normal general purpose I/O pin function.
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 register forms a buffer between the inter-
nal bus and the transmitter shift register. It should be noted that if 9-bit data format has been se-
lected, 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 and not as an I/O
Access the USRregister and write the data that is to be transmitted into the TXR 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 register.
Clearing the TXIF flag is always achieved using the following software sequence:
1. A USR register access
2. A TXR register write execution
The read-only TXIF flag is set by the UART hardware and if set indicates that the TXR register is
empty and that other data can now be written into the TXR register without overwriting the previ-
ous data. If the TEIE bit is set then the TXIF flag will generate an interrupt.
During a data transmission, a write instruction to the TXR register will place the data into the TXR
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 TXR register will place the data
directly into the shift register, resulting in the commencement of data transmission, and the TXIF
bit being immediately set. When a frame transmission is complete, which happens after stop bits
are sent or after the break frame, the TIDLE bit will be set. To clear the TIDLE bit the following soft-
ware sequence is used:
1. A USR register access
2. A TXR register write execution
Note that both the TXIF and TIDLE bits are cleared by the same software sequence.
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