Download Microcontroller Systems ELET 3232 Topic 19: Hardware Design Concepts #1

Microcontroller Systems
ELET 3232
Topic 19: Hardware Design Concepts #1
Power Supplies, Power-On Reset and Clocks
1
Objectives

To design and implement three hardware
circuits common to all embedded controllers

Power Sources





Rectified AC
Battery
PC Port Signals
Clock Generation
Power-On Reset
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Power Sources

Power Sources
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
Critical component of the embedded controller circuit
Various options should be considered

Will the system be portable?


Will the system be a stand-alone device?


Battery Powered
Need an AC wall socket for power?
Computer Add-On

Can we use various port pins on the PC to get the required
power
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Batteries

Capacity: how long will the battery last?



Voltage:


Typically VCC must be between 2.7v and 5.5v
Storage:


mA hr rating
Capacity may dictate size
Recommended storage method when not in use
Shelf Life:

How long will it last before completely discharging
on its own
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Batteries

Operating Temperature:



Duty Cycle:



Batteries typically have poor temperature characteristics
Rely on chemical reactions that are directly affected by
temperature
Can the battery be used continuously
Some perform better when periodically turned off
Chargeable:

What type of battery:


Lithium Ion
NiCd
5
Batteries
3.7v, 50 A hr
$249.95
8.4v, 400 mA hr
$7.95
1.2v, 700 mA hr
$0.89
3.6v, 40 mA hr
$3.95
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Main Power Supply

3 VDC @ 200 mA Adapter
Input: 117 VAC, 60 Hz, 5 watt
$1.75
Use wall sockets:



120 VAC
Step-Down to about 6 VAC
Rectified and filtered

Wall units are commonly used
5 VDC @ 2.4 Amp Regulated, Filtered and Grounded
Input: 100 - 125 VAC, 47 - 63 Hz, 0.4 A
$9.95
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Main Power Supply

Simple filtered rectifier


VAC should be stepped-down to about 6VAC
Embedded system could use from 200mA – 500mA


Diodes should be rated at 1A
With 6 to 12 VAC input

Rule of thumb:
 PIV
PIVshould be a minimum
of 3x input voltage
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Main Power Supply

Voltage Regulators are commonly used


Provide a stable voltage
7805 is a common 3 terminal regulator



Can accept 9 to 20 VDC
Provides 1 A of current
Common regulator circuit using the LM1117:
9
PC Port Signals


Personal Computers typically have serial and
USB ports
Older PCs may have parallel ports


Signals (pins) on these ports can be used to supply
power to an embedded controller board
RS232 (serial port) signals can usually provide
about 10mA each
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RS232 Port Signals

Can use:


TxD (Transmit Data): when idle – will have -12v
DTR (Data Terminal Ready):



could have ± 12v
The voltage drops when the line is loaded
RTS (Request to Send): same as DTR
5.1 VDC
11
Clock Generation

VCXO (Voltage Controlled Crystal Oscillators)




IC available in 8 or 14 pin DIP or Surface Mount
Clock signal is applied to X1 input
Supply voltage required for operation
Advantages:



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Industry standard packages
CMOS/TTL Output
3.3 v operation available
Large fanout capability
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Clock Generation

Crystal resonators:


X1
X2
Low-cost
Advantages


No additional capacitors needed
Wide frequency range available


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

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2 to 60 MHz
ftolerance = .5%
fstability = .3%
faging = .3% over ten years
No external adjustment required
Small mechanical profile
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Clock Generation
X1

X2
Quartz Crystals

Most popular option



Widely available
Dependable
Requires two additional capacitors
14
Power-On Reset

The simplest power-on reset circuit is shown
below



It may not function correctly
AVR needs to have the RESET line low for a
specified period of time for the system to be reset
The capacitor may allow the RESET line to go high
too quickly
RESET
15
Power-On Reset

It is common to include the resistors


Value of RC will determine how long
RESET “sees” a low logic level
Example: if C13=100pfd and R1 is 33 kΩ
as shown


RC = 3.3 μS
RESET would “see” a low logic for roughly 2
or 3 time constants – 6.6 to 10 μS
16
Power-On Reset

A timer circuit may also be used if precision is
required
17
Summary

In this topic we designed three hardware
circuits common to all embedded controllers

Power Sources





Rectified AC
Battery
PC Port pins
Clock Generation
Power-On Reset
18