Motorola MCF5281 Uart Receive Buffers URBn, Uart Transmit Buffers UTBn, UCRn Field Descriptions

Models: MCF5282 MCF5281

1 816
Download 816 pages 28.97 Kb
Page 485
Image 485

 

 

 

Register Descriptions

 

 

Table 23-6. UCRn Field Descriptions (continued)

 

 

 

Bits

Value

Command

Description

 

 

 

 

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.

 

 

 

 

 

01

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.

 

 

 

 

 

10

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.

 

 

 

 

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.

Field

Reset

R/W

Address

7

0

RB

1111_1111

Read only

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

Figure 23-7. UART Receive Buffer (URBn)

23.3.7 UART Transmit Buffers (UTBn)

The transmit buffers consist of the transmitter holding register and the transmitter shift register. The holding register accepts characters from the bus master if channel’s 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.

MOTOROLA

Chapter 23. UART Modules

23-11

Page 485
Image 485
Motorola MCF5281, MCF5282 user manual Uart Receive Buffers URBn, Uart Transmit Buffers UTBn, UCRn Field Descriptions