DAN-176 Datasheet, PDF (3/5 Pages) Exar Corporation – POWER-SAVE FEATURE: SLEEP CURRENT DOWN

DATA COMMUNICATIONS APPLICATION NOTE
DAN176
5.0 DATA LOSS DURING SLEEP/POWER-SAVE
When the Low Power UART has entered Sleep or Power-Save mode, the oscillator is shut off to conserve
power. It takes up to tens of milliseconds to re-start the oscillator when a crystal is used to provide the UART
clock. Therefore, an incoming character that is used to wake up the UART may not get assembled correctly be-
cause of this delay. On the other hand, the oscillator/buffer starts up immediately (within a few nanoseconds)
when an external clock is used to provide the UART clock and is not shut off during Sleep/Power-Save mode.
In applications where an incoming character on the RX pin will be used to wake up the UART, it is recommend-
ed to use an external clock and keep it running during Sleep/Power-Save mode so that the first character re-
ceived will get assembled correctly. This will prevent any data loss without compromising the low power con-
sumption during Power-Save mode.
FIGURE 3. APPLICATION EXAMPLE USING POWER-SAVE FEATURE
CPU/FPGA
Address A2:A0
Databus D7:D0
CS#
IOR#
IOW#
XR16L580IL
XR16L2551IL
XR16L2751IM
I/O
PwrSave
XTAL1
Typical Clock Frequency of
1.8432MHz to 14.7456MHz
External
Crystal
Oscillator
XTAL2
N.C.
RS-232,
RS-485, or
Infrared
Applications:
Handheld, Portable
and Mobile Devices
such as PDA’s, Tablet
PC’s, IR Scanners etc.
3