XR16C850CM-F Datasheet, PDF (23/56 Pages) Exar Corporation – 2.97V TO 5.5V UART WITH 128-BYTE FIFO

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REV. 2.3.1
XR16C850
2.97V TO 5.5V UART WITH 128-BYTE FIFO
2.21 Sleep Mode with Auto Wake-Up
The 850 supports low voltage system designs, hence, a sleep mode is included to reduce its power
consumption when the chip is not actively used.
All of these conditions must be satisfied for the 850 to enter sleep mode:
■ no interrupts pending for the 850 (ISR bit-0 = 1)
■ sleep mode is enabled (IER bit-4 = 1)
■ modem inputs are not toggling (MSR bits 0-3 = 0)
■ RX input pin is idling at a logic 1
The 850 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 850 resumes normal operation by any of the following:
■ a receive data start bit transition (logic 1 to 0)
■ 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 850 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 850 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 850 will stay in the sleep mode of operation until it is disabled by setting IER bit-4 to a
logic 0.
If the address lines, data bus lines, IOW#, IOR#, CSA#, CSB#, and modem input lines remain steady when the
850 is in sleep mode, the maximum current will be in the microamp range as specified in the DC Electrical
Characteristics on page 42. If the input lines are floating or are toggling while the 850 is in sleep mode, the
current can be up to 100 times more. If any of those signals are toggling or floating, then an external buffer
would be required to keep the address, data and control lines steady to achieve the low current.
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. Also, make sure the RX input is idling at logic 1 or “marking” condition
during sleep mode. This may not 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 system design
engineer can use a 47k ohm pull-up resistor on the RX input.
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