XR16L651 Datasheet, PDF (21/56 Pages) Exar Corporation – 2.25V TO 5.5V UART WITH 32-BYTE FIFO

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REV. 1.3.0
XR16L651
2.25V TO 5.5V UART WITH 32-BYTE FIFO
2.17 Sleep Mode & Wake-up Indicator
The 651 is designed to operate with low power consumption. A special sleep mode is included to further
reduce power consumption when the chip is not being used. All of these conditions must be satisfied for the
651 to enter sleep mode:
s no interrupts pending 651 (ISR bit-0 = 1)
s sleep mode is enabled (IER bit-4 = 1)
s modem inputs are not toggling (MSR bits 0-3 = 0)
s RX input pin is idling at a logic 1
The 651 stops its crystal oscillator to conserve power in the sleep mode. User can check the XTAL2 pin for no
clock output as an indication that the device has entered the sleep mode.
The 651 resumes normal operation by any of the following:
s a receive data start bit transition (logic 1 to 0)
s a data byte is loaded to the transmitter, THR or FIFO
s a change of logic state on any of the modem or general purpose serial inputs: CTS#, DSR#, CD#, RI#
If the sleep mode is enabled and the 651 is awakened by one of the conditions described above, an interrupt is
issued by the 651 to signal to the CPU that it is awake. The lower nibble of the interrupt source register (ISR)
will read a value of 0x1 for this interrupt and reading the ISR clears this interrupt. Since the same value (0x1) is
also used to indicate no pending interrupt, users should exercise caution while using the sleep mode. Once
awakened, the 651 will return to the sleep mode automatically after any other interrupting condition (the true
cause of waking up the 651) has been serviced. If the 651 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 651 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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