XR20V2170 Datasheet, PDF (18/52 Pages) Exar Corporation – I2C/SPI UART WITH 64-BYTE FIFO AND RS232 TRANSCEIVER

XR20V2170
I2C/SPI UART WITH 64-BYTE FIFO AND RS232 TRANSCEIVER
REV. 1.0.0
The V2170 UART portion 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 V2170 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: CTS#, DSR#, CD#, RI#
If the UART portion of V2170 is awakened by any one of the above conditions, it will return to the sleep mode
automatically after all interrupting conditions have been serviced and cleared. If the UART portion of V2170 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 V2170 will stay in the sleep
mode of operation until it is disabled by setting IER bit-4 to a logic 0.
2.15.1.2 UART active, charge pump of RS-232 transceiver shut down
If the ACP pin is HIGH and the UART portion of the V2170 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) are LOW
■ modem inputs have been idle for approximately 30 seconds
When these conditions are satisfied, the V2170 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 V2170
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.
2.15.2 Full Sleep Mode
In full sleep mode, the V2170 shuts down the charge pump and the internal oscillator. The V2170 enters the full
sleep mode if the following conditions are satisfied:
■ the UART portion of the V2170 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 V2170 can be accessed. The
V2170 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.
If the serial clock, serial data, and modem input lines remain steady when the V2170 is in sleep mode, the
maximum current will be in the microamp range as specified in the DC Electrical Characteristics on page 38.
A word of caution: owing to the starting up delay of the crystal oscillator after waking up from sleep mode, the
first few receive characters may be lost. The number of characters lost during the restart also depends on your
operating data rate. More characters are lost when operating at higher data rate. Also, it is important to keep
RX input idling HIGH or “marking” condition during sleep mode to avoid receiving a “break” condition upon the
restart. This may occur when the external interface transceivers (RS-232, RS-485 or another type) are also put
to sleep mode and cannot maintain the “marking” condition. To avoid this, the designer can use a 47k-100k
ohm pull-up resistor on the RX input pin.
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