DAN-176 Datasheet, PDF (1/5 Pages) Exar Corporation – POWER-SAVE FEATURE: SLEEP CURRENT DOWN

DATA COMMUNICATIONS APPLICATION NOTE
DAN176
August 2004
POWER-SAVE FEATURE: SLEEP CURRENT DOWN TO 15µA
Author: BL
1.0 INTRODUCTION
This application note describes the Power-Save feature of Exar’s single channel XR16L580 (L580) UART and
the two channel XR16L2551 (L2551) and XR16L2751 (L2751) UARTs. These three devices will be collectively
referred to as the Low Power UART in this application note. Please refer to individual device datasheets for
specific information.
2.0 SLEEP MODE
The Low Power UART includes a sleep mode that reduces power consumption when the device is not actively
used. The Low Power UART stops its clock oscillator to conserve power in the sleep mode. However, the ad-
dress lines, the databus and the control lines (chipselect, read and write strobes) are still active during sleep
mode so that the internal registers of the device can be accessed. These signals (except the chipselect) are
typically shared among many devices in the system. Any activity on these signals will translate into increased
power drain from the Low Power UART thereby defeating the purpose of the sleep mode. The Low Power
UART’s Power-Save feature resolves this problem.
3.0 POWER-SAVE FEATURE
The Low Power UART has a Power-Save mode which further reduces the power consumption in sleep mode
by isolating the device from the databus interface. In this mode, the power consumption is steady (in the range
15 - 50µA at 3.3V) and is not affected by any activity on the databus, address or control lines. However, the in-
ternal registers of the device cannot be accessed while in Power-Save mode. Figure 1 below shows the block
diagram of the Low Power UART. The L2551 and L2751 are very similar to the L580, the difference being the
extra CS# and INT lines for the second UART channel. The following section describes how the Low Power
device can be programmed in and out of the Sleep and Power-Save states.
FIGURE 1. BLOCK DIAGRAM SHOWING THE POWER-SAVE FEATURE OF THE Low Power UART
PwrSave
A2:A0
D7:D0
IOR#
IOW# (R/W#)
CS#
INT (IRQ#)
RESET
(RESET#)
16/68#
Intel or
Motorola
Data Bus
Interface
*5 V Tolerant Inputs
UART BLOCK
UART BLOCK
Crystal Osc/Buffer
VCC
(2.25 to 5.5 V)
GND
RX
CTS#
DSR#
CD#
RI#
TX
RTS#
DTR#
XTAL1
XTAL2
GNugget_BLK
EXAR Corporation 48720 Kato Road, Fremont CA, 94538 • [phone] • FAX [phone] • www.exar.com • [e-mail]