XR19L220_07 Datasheet, PDF (15/43 Pages) Exar Corporation – SINGLE CHANNEL INTEGRATED UART AND RS-232 TRANSCEIVER

XR19L220
REV. 1.0.1
SINGLE CHANNEL INTEGRATED UART AND RS-232 TRANSCEIVER
The L220 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 L220 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
■ a change of logic state on any of the modem or general purpose serial inputs: i.e., any of the MSR bits 0-
3 shows a ’1’
The UART portion in the L220 will return to the sleep mode automatically after all interrupting conditions have
been serviced and cleared. If the UART portion of the L220 is awakened by the modem inputs, a read to the
MSR is required to reset the modem inputs. In any case, the sleep mode will not be entered while an interrupt
is pending. The UART portion of the L220 will stay in the sleep mode of operation until it is disabled by setting
IER bit-4 to a logic 0.
2.17.1.2 UART active, charge pump of RS-232 transceiver shut down
If the ACP pin is HIGH and the UART portion of the L220 is not in sleep mode, then the charge pump will
automatically shut down to conserve power if the following conditions are true:
■ no activity on the TXD output signal
■ modem input signals (RX, CTS, DSR, CD, RI) are LOW
■ modem inputs have been idle for approximately 30 seconds
When these conditions are satisfied, the L220 shuts down the charge pump and tri-states the RS-232 drivers
to conserve power. In this mode, the RS-232 receivers are fully active and the internal registers of the L220
can be accessed. The time for the charge pump to resume normal operation after exiting the sleep mode is
typically 45µs. It will wake up 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
■ a LOW to HIGH transition on any of the modem or general purpose serial inputs
Because the receivers are fully active when the charge pump is turned off, any data received will be transferred
to/from the UART without any issues.
2.17.2 Full Sleep Mode
In full sleep mode, the L220 shuts down the charge pump and the internal oscillator. The L220 enters the full
sleep mode if the following conditions are satisfied:
■ the UART portion of the L220 is already in sleep mode (no output on XTAL2)
■ the ACP (Autosleep for Charge Pump) pin is HIGH
When these conditions are satisfied, both the UART and the charge pump will be in the sleep mode. In this
mode, the RS-232 receivers are fully active and the internal registers of the L220 can be accessed. The L220
exits the full sleep mode if either the ACP pin becomes LOW or the internal oscillator starts up. The time for the
charge pump to resume normal operation after exiting the full sleep mode is typically 45µs.
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