Operation

from the CPU to a serial bit stream on UTXDn. It automatically sends a start bit followed by the programmed number of data bits, an optional parity bit, and the programmed number of stop bits. The lsb is sent first. Data is shifted from the transmitter output on the falling edge of the clock source.

After the stop bits are sent, if no new character is in the transmitter holding register, the UTXDn output remains high (mark condition) and the transmitter empty bit, USRn[TxEMP], is set. Transmission resumes and TxEMP is cleared when the CPU loads a new character into the UART transmit buffer (UTBn). If the transmitter receives a disable command, it continues until any character in the transmitter shift register is completely sent.

If the transmitter is reset through a software command, operation stops immediately (see Section 23.3.5, “UART Command Registers (UCRn)”). The transmitter is reenabled through the UCRn to resume operation after a disable or software reset.

If the clear-to-send operation is enabled, UCTSn must be asserted for the character to be transmitted. If UCTSn is negated in the middle of a transmission, the character in the shift register is sent and UTXD remains in mark state until UCTSn is reasserted. If the transmitter is forced to send a continuous low condition by issuing a SEND BREAK command, the transmitter ignores the state of UCTSn.

If the transmitter is programmed to automatically negate URTSn when a message transmission completes, URTSn must be asserted manually before a message is sent. In applications in which the transmitter is disabled after transmission is complete and URTSn is appropriately programmed, URTSn is negated one bit time after the character in the shift register is completely transmitted. The transmitter must be manually reenabled by reasserting URTSn before the next message is to be sent.

Figure 23-20shows the functional timing information for the transmitter.

MOTOROLA

Chapter 23. UART Modules

23-21

Page 495
Image 495
Motorola MCF5281, MCF5282 user manual 23-21