REV. 1.1.3 2.15 Auto Xon/Xoff (Software) Flow Control When software flow control" />
參數(shù)資料
型號(hào)： XR16L2550IM-F
廠商： Exar Corporation
文件頁(yè)數(shù)： 8/44頁(yè)
文件大?。?/td> 0K
描述： IC UART FIFO 16B DUAL 48TQFP
標(biāo)準(zhǔn)包裝： 250
特點(diǎn)： *
通道數(shù)： 2，DUART
FIFO's： 16 字節(jié)
規(guī)程： RS232
電源電壓： 2.25 V ~ 5.5 V
帶自動(dòng)流量控制功能： 是
帶IrDA 編碼器/解碼器： 是
帶故障啟動(dòng)位檢測(cè)功能： 是
帶調(diào)制解調(diào)器控制功能： 是
帶CMOS： 是
安裝類型： 表面貼裝
封裝/外殼： 48-TQFP
供應(yīng)商設(shè)備封裝： 48-TQFP（7x7）
包裝： 托盤
其它名稱： 1016-1278
XR16L2550
16
LOW VOLTAGE DUART WITH 16-BYTE FIFO
REV. 1.1.3
2.15
Auto Xon/Xoff (Software) Flow Control
When software flow control is enabled (See Table 12), the L2550 compares one or two sequential receive data
characters with the programmed Xon or Xoff-1,2 character value(s). If receive character(s) (RX) match the
programmed values, the L2550 will halt transmission (TX) as soon as the current character has completed
transmission. When a match occurs, the Xoff (if enabled via IER bit-5) flag will be set and the interrupt output
pin will be activated. Following a suspension due to a match of the Xoff character, the L2550 will monitor the
receive data stream for a match to the Xon-1,2 character. If a match is found, the L2550 will resume operation
and clear the flags (ISR bit-4).
Reset initially sets the contents of the Xon/Xoff 8-bit flow control registers to a logic 0. Following reset the user
can write any Xon/Xoff value desired for software flow control. Different conditions can be set to detect Xon/
Xoff characters (See Table 12) and suspend/resume transmissions. When double 8-bit Xon/Xoff characters
are selected, the L2550 compares two consecutive receive characters with two software flow control 8-bit
values (Xon1, Xon2, Xoff1, Xoff2) and controls TX transmissions accordingly. Under the above described flow
control mechanisms, flow control characters are not placed (stacked) in the user accessible RX data buffer or
FIFO.
In the event that the receive buffer is overfilling and flow control needs to be executed, the L2550 automatically
sends an Xoff message (when enabled) via the serial TX output to the remote modem. The L2550 sends the
Xoff-1,2 characters two-character-times (= time taken to send two characters at the programmed baud rate)
after the receive FIFO crosses the programmed trigger level. To clear this condition, the L2550 will transmit the
programmed Xon-1,2 characters as soon as receive FIFO is less than one trigger level below the programmed
trigger level. See Table 6 below.
TABLE 6: AUTO XON/XOFF (SOFTWARE) FLOW CONTROL
RX TRIGGER LEVEL
INT PIN ACTIVATION
XOFF CHARACTER(S) SENT
(CHARACTERS IN RX FIFO)
XON CHARACTER(S) SENT
(CHARACTERS IN RX FIFO)
1
1*
0
4
4*
1
8
8*
4
14
14*
8
* After the trigger level is reached, an xoff character is sent after a short span of time (= time required to send 2
characters); for example, after 2.083ms has elapsed for 9600 baud and 8-bit word length, no parity and 1 stop bit setting.
2.16
Special Character Detect
A special character detect feature is provided to detect an 8-bit character when bit-5 is set in the Enhanced
Feature Register (EFR). When this character (Xoff2) is detected, it will be placed in the FIFO along with normal
incoming RX data.
The L2550 compares each incoming receive character with Xoff-2 data. If a match exists, the received data will
be transferred to FIFO and ISR bit-4 will be set to indicate detection of special character. Although the Internal
Register Table shows Xon, Xoff Registers with eight bits of character information, the actual number of bits is
dependent on the programmed word length. Line Control Register (LCR) bits 0-1 defines the number of
character bits, i.e., either 5 bits, 6 bits, 7 bits, or 8 bits. The word length selected by LCR bits 0-1 also
determines the number of bits that will be used for the special character comparison. Bit-0 in the Xon, Xoff
Registers corresponds with the LSB bit for the receive character.
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