110118-0001A
43
In Sleep mode, sometimes called "Suspend" mode, the processor puts the SDRAM in a low-
power, self-refresh mode, the processor core shuts off, most peripheral sub-systems are shut down
and the power supplies drop into low-power states or turn off entirely (see the diagram in section
5.3.2 for details). In this state, the BitsyXb consumes very little power, most of which is dedicated
to the maintenance of the RAM (see section 6.3.5 for specifications). The system can be
"awakened" and returned to the Run state by initiating a system wakeup using one of the methods
described in section 5.3.3.
If Main Power (DCIN_POS or VBATT_POS) drops below the power supply trip point (section
6.3.2), the BitsyXb automatically goes in Sleep mode. The transition time from Run mode to
Sleep mode is a function of the operating system. System Sleep can also be initiated
programmatically.
5.2
Architectural Overview and Power Management Features
This section provides an overview of the architecture of the BitsyXb power supply and a
description of the various features of the BitsyXb power management systems.
5.2.1
Power Supply Architecture
The BitsyXb power supply is laid out as shown in the following diagram. Incoming DC power is
regulated to 5 and 3.3 V. Other system voltages are derived from these power supplies.
The following diagram is for the BitsyX. BitsyXb changes include external supercaps and Vee
supply.
5 V
Vddi
to CPU core
DCIN_POS
VBATT_POS
Vee
to display
Vcc
supercaps
Vddi
Vee
3.3 V
switcher
Vddx
linear
DCIN_POS and VBATT_POS voltages are mixed using a diode with a low forward voltage. This
allows a battery and DC power supply to be connected at the same time. If only one power supply
is used for your system, use the VBATT_POS input. See sections 5.3.7 and 5.5.3 for examples.
Vddi is a variable-voltage power supply controlled by the processor I
2
C bus (4.5.4). This voltage
scaling feature allows the operating system to manage power consumption over the full range of
CPU clock rates.
