XR16L784_08 Datasheet, PDF (17/51 Pages) Exar Corporation – HIGH PERFORMANCE 2.97V TO 5.5V QUAD UART

XR16L784
REV. 1.2.3
HIGH PERFORMANCE 2.97V TO 5.5V QUAD UART
2.15 Sleep Mode with Wake-Up Indicator
The 784 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 784 to enter sleep mode:
■ no interrupts pending for all 8 channels of the 784 (ISR bit-0 = 1)
■ SLEEP register = 0x0F
■ modem inputs are not toggling (MSR bits 0-3 = 0)
■ RX input pin of all 4 channels are idling HIGH
The 784 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 784 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 784 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 784 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. Also, a special interrupt is generated with an indication of no pending
interrupt. Reading INT0 will clear this special interrupt. The 784 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.
17