XR16M698 datasheet(21/58 Pages) EXAR

21 / 58 page

XR16M698

REV. 1.0.0

1.62V TO 3.63V HIGH PERFORMANCE OCTAL UART WITH 32-BYTE FIFO

2.15

Sleep Mode with Auto Wake-Up

The 698 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 698 to enter sleep mode:

■ no interrupts pending for all 8 channels of the 698 (ISR bit-0 = 1)

■ SLEEP register = 0xFF

■ modem inputs are not toggling (MSR bits 0-3 = 0)

■ RX input pin of all 8 channels are idling HIGH

The 698 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 698 resumes normal operation by any of the following:

■ a receive data start bit transition (HIGH to LOW)

■ 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 698 is awakened by any one of the above conditions, it will generate an interrupt. If the interrupt for the

event that woke up the 698 is not enabled, then a special wake-up interrupt occurs where reading the interrupt

status register will return a "no interrupt" indication. For example, there is a change of state on the CTS# input

that wakes up the 698, but the MSR interrupt is not enabled. Reading the interrupt status register will return a

value indicating that there are no pending interrupts and will clear the wake-up interrupt.

The 698 will return to the sleep mode automatically after all interrupting conditions have been serviced and

cleared. If the 698 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 in any channel. The 698 will stay

in the sleep mode of operation until it is disabled by setting SLEEP = 0x00.

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.

XR16M698 Datasheet(PDF) 21 Page - Exar Corporation