Microcontroller User's Manual

MOTOROLA Chapter 23. UART Modules 23-11

23.3.6 UART Receive Buffers (URBn)

The receive buffers contain one serial shift register and three receiver holding registers,

which act as a FIFO. RXD is connected to the serial shift register. The CPU reads from the

top of the stack while the receiver shifts and updates from the bottom when the shift register

is full (see Figure 23-19). RB contains the character in the receiver.

23.3.7 UART Transmit Buffers (UTBn)

The transmit buffers consist of the transmitter holding register and the transmitter shift

USRn[TxRDY] is set. A write to the transmit buffer clears USRn[TxRDY], inhibiting any

more characters until the shift register can accept more data. When the shift register is

empty, it checks if the holding register has a valid character to be sent (TxRDY = 0). If there

is a valid character, the shift register loads it and sets USRn[TxRDY] again. Writes to the

transmit buffer when the channel’s TxRDY = 0 and when the transmitter is disabled have

no effect on the transmit buffer.

1–0

RC (This field selects a single command)

00 NO ACTION TAKEN Causes the receiver to stay in its current mode. If the receiver is enabled, it remains

enabled; if disabled, it remains disabled.

RECEIVER ENABLE If the UART module is not in multidrop mode (UMR1n[PM] ≠ 11), RECEIVER ENABLE

enables the channel's receiver and forces it into search-for-start-bit state. If the receiver is

already enabled, this command has no effect.

RECEIVER DISABLE Disables the receiver immediately. Any character being received is lost. The command

does not affect receiver status bits or other control registers. If the UART module is

programmed for local loop-back or multidrop mode, the receiver operates even though this

command is selected. If the receiver is already disabled, the command has no effect.

11 — Reserved, do not use.

7 0

Field RB

Reset 1111_1111

R/W Read only

Address IPSBAR + 0x20C(URB0), 0x24C(URB1), 0x28C(URB2)

Figure 23-7. UART Receive Buffer (URBn)

Table 23-6. UCRn Field Descriptions (continued)

Bits Value Command Description