XR19L200 Datasheet, PDF (12/40 Pages) Exar Corporation – SINGLE CHANNEL INTEGRATED UART AND RS-232 TRANSCEIVER

XR19L200
PRELIMINARY
SINGLE CHANNEL INTEGRATED UART AND RS-232 TRANSCEIVER
REV. P1.0.2
TABLE 4: AUTO XON/XOFF (SOFTWARE) FLOW CONTROL
RX TRIGGER LEVEL INT PIN ACTIVATION
1
1
4
4
8
8
14
14
XOFF CHARACTER(S) SENT
(CHARACTERS IN RX FIFO)
1*
4*
8*
14*
XON CHARACTER(S) SENT
(CHARACTERS IN RX FIFO)
0
1
4
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.
2.13 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 L200 compares each incoming receive character with the programmed Xoff-2 data. If a match exists, the
received data will be transferred to the RX 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.
2.14 Sleep Modes with Wake-Up Interrupt
There are two levels of power management integrated in the L200. The device is low power with low
operational and standby supply currents. In the Partial Sleep mode, the internal oscillator of the UART or
charge pump of the RS-232 transceiver is turned off to reduce the power consumption. In the Full Sleep mode,
both the oscillator and the charge pump are turned off.
2.14.1 Partial Sleep Mode
There are two different partial sleep modes. In the first mode, the UART is in sleep mode and the RS-232
transceiver is active. In the other mode, the UART is active but the charge pump of the RS-232 transceiver is
turned off.
2.14.1.1 UART in sleep mode, RS-232 transceiver active
If the ACP pin is LOW, then the charge pump for the RS-232 transceiver will always be active. But the UART
portion in the L200 can still enter sleep mode if all of these conditions are satisfied:
■ no interrupts pending (ISR bit-0 = 1)
■ the 16-bit divisor programmed in DLM and DLL registers is a non-zero value
■ sleep mode is enabled (IER bit-4 = 1)
■ RXD input pin is idling LOW
The L200 stops its crystal oscillator to conserve power in this mode. The user can check the XTAL2 pin for no
clock output as an indication that the device has entered the partial sleep mode.
The UART portion in the L200 resumes normal operation or active mode by any of the following:
■ a receive data start bit transition on the RXD input (LOW to HIGH)
■ a data byte is loaded to the transmitter, THR or FIFO
The UART portion in the L200 will return to the sleep mode automatically after all interrupting conditions have
been serviced and cleared. In any case, the sleep mode will not be entered while an interrupt is pending. The
UART portion of the L200 will stay in the sleep mode of operation until it is disabled by setting IER bit-4 to a
logic 0.
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