Embedded Systems Design with the Atmel AVR Microcontroller Part II

7.2 BATTERY OPERATION
7.2. BATTERY OPERATION 185
Many embedded systems are remote, portable systems operating from a battery supply. To properly
design a battery source for an embedded system, the operating characteristics of the embedded
system must be matched to the characteristics of the battery supply.
7.2.1 EMBEDDED SYSTEM VOLTAGE AND CURRENT DRAIN SPECIFICA-
TIONS
An embedded system has a required supply voltage and an overall current requirement. For the
purposes of illustration, we will assume our microcontroller based embedded system operates from
5 VDC. The overall current requirements of the system is determined by the worst case current
requirements when all embedded system components are operational.
7.2.2 BATTERY CHARACTERISTICS
To properly match a battery to an embedded system, the battery voltage and capacity must be
specified. Battery capacity is typically specified as a mAH rating. For example, a typical 9 VDC nonrechargeable
alkaline battery has a capacity of 550 mAH. If the embedded system has a maximum
operating current of 50 mA, it will operate for approximately eleven hours before battery replacement
is required.
A battery is typically used with a voltage regulator to maintain the voltage at a prescribed
level. Figure 7.2 provides sample circuits to provide a +5 VDC and a ±5 VDC portable battery
source. Additional information on battery capacity and characteristics may be found in Barrett and
Pack (S. Barrett, 2004).
7.3 INPUT DEVICES
In this section, we discuss how to properly interface input devices to a microcontroller. We will start
with the most basic input component, a simple on/off switch.
7.3.1 SWITCHES
Switches come in a variety of types. As a system designer it is up to you to choose the appropriate
switch for a specific application. Switch varieties commonly used in microcontroller applications are
illustrated in Figure 7.3(a). Here is a brief summary of the different types:
Slide switch: A slide switch has two different positions: on and off. The switch is manually
moved to one position or the other.For microcontroller applications,slide switches are available
that fit in the profile of a common integrated circuit size dual inline package (DIP). A bank
of four or eight DIP switches in a single package is commonly available.
Momentary contact pushbutton switch: A momentary contact pushbutton switch comes
in two varieties: normally closed (NC) and normally open (NO). A normally open switch,