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參數(shù)資料
| 型號(hào)： | LPC47B34X |
| 廠商： | STANDARD MICROSYSTEMS CORP |
| 元件分類： | 外設(shè)及接口 |
| 英文描述： | 128 Pin Enhanced Super I/O with LPC Interface for Consumer Applications |
| 中文描述： | MULTIFUNCTION PERIPHERAL, PQFP128 |
| 封裝： | QFP-128 |
| 文件頁數(shù)： | 90/250頁 |
| 文件大?。?/td> | 645K |
| 代理商： | LPC47B34X |
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90
Notes On Serial Port Operation
FIFO Mode Operation
GENERAL
The RCVR FIFO will hold up to 16 bytes regardless of which trigger level is selected.
TX AND RX FIFO Operation
The Tx portion of the UART transmits data through TXD as soon as the CPU loads a byte into the Tx
FIFO.
The UART will prevent loads to the Tx FIFO if it currently holds 16 characters.
Loading to
the Tx FIFO will again be enabled as soon as the next character is transferred to the Tx shift register.
These capabilities account for the largely autonomous operation of the Tx.
The UART starts the above operations typically with a Tx interrupt. The chip issues a Tx interrupt
whenever the Tx FIFO is empty and the Tx interrupt is enabled, except in the following instance.
Assume that the Tx FIFO is empty and the CPU starts to load it. When the first byte enters the FIFO
the Tx FIFO empty interrupt will transition from active to inactive. Depending on the execution speed
of the service routine software, the UART may be able to transfer this byte from the FIFO to the shift
register before the CPU loads another byte. If this happens, the Tx FIFO will be empty again and
typically the UART's interrupt line would transition to the active state. This could cause a system with
an interrupt control unit to record a Tx FIFO empty condition, even though the CPU is currently
servicing that interrupt.
Therefore, after the first byte has been loaded into the FIFO the UART
will wait one serial character transmission time before issuing a new Tx FIFO empty interrupt.
This one character Tx interrupt delay will remain active until at least two bytes have the Tx
FIFO empties after this condition, the Tx been loaded into the FIFO, concurrently. When
interrupt will be activated without a one character delay.
Rx support functions and operation are quite different from those described for the transmitter. The
Rx FIFO receives data until the number of bytes in the FIFO equals the selected interrupt trigger level.
At that time if Rx interrupts are enabled, the UART will issue an interrupt to the CPU. The Rx FIFO
will continue to store bytes until it holds 16 of them. It will not accept any more data when it is full.
Any more data entering the Rx shift register will set the Overrun Error flag. Normally, the FIFO depth
and the programmable trigger levels will give the CPU ample time to empty the Rx FIFO before an
overrun occurs.
One side-effect of having a Rx FIFO is that the selected interrupt trigger level may be above the data
level in the FIFO. This could occur when data at the end of the block contains fewer bytes than the
trigger level. No interrupt would be issued to the CPU and the data would remain in the UART.
To
prevent the software from having to check for this situation the chip incorporates a timeout
interrupt.
The timeout interrupt is activated when there is a least one byte in the Rx FIFO, and neither the CPU
nor the Rx shift register has accessed the Rx FIFO within 4 character times of the last byte. The
timeout interrupt is cleared or reset when the CPU reads the Rx FIFO or another character enters it.
These FIFO related features allow optimization of CPU/UART transactions and are especially useful
given the higher baud rate capability (256 kbaud).
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