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Download Microcontroller Systems ELET 3232 Topic 19: Hardware Design Concepts #1
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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 2 Power Sources Power Sources 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 3 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 4 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 6 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 7 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 8 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 10 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: Industry standard packages CMOS/TTL Output 3.3 v operation available Large fanout capability 12 Clock Generation Crystal resonators: X1 X2 Low-cost Advantages No additional capacitors needed Wide frequency range available 2 to 60 MHz ftolerance = .5% fstability = .3% faging = .3% over ten years No external adjustment required Small mechanical profile 13 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