Laboratory Exercise: Power and Energy Efficiency in Embedded Systems
Learning Outcomes:
1: Understand appropriate techniques in smart and embedded systems engineering.
2: Explain the sustainability factors in electronic design.
3: Apply relevant technologies to smart embedded systems.
4: Implement smart embedded systems and evaluate their performance.
The following 4 questions to be answered for the final submission of CW2.
Question 1: Assuming the main oscillator in an LPC1768 application is running at 16.000 MHz, the PLL multiplies by 8, and the lower 7 bits of register CCLKCFG are set to 5. What is the frequency of cclk?
Question 2: With the aid of a diagram describe the functions of the three clock sources within the LPC1768. (Word limit not more than 300)
Question 3: Propose and discuss practical situations where each of the LPC1768 low power modes (Sleep, Deep Sleep, Power Down and Deep Power Down) can be used effectively.
(Word limit not more than 500)
Question 4: Critically analysis dynamic voltage and frequency scaling; describe their principles and how they can be used to efficiently to reduce power consumption in embedded designs; compare and contrast them by analyzing their advantages and disadvantages; give practical examples in an embedded system context.
(This should not exceed 1000 words. Use numbered citations in your text, which should be between 2 and 5 citations. Figures and Tables can be used.)
Assignment Task 1: Changing the CPU Clock Divider settings
Compile and download Program Example 1 above to an mbed, first with the lineCCLKCFG=0x00000005;commented out. The clock frequency will not be changed.
Carefullyrecord how many times the led flashes in 30 s. Now enable the divider code line, andrun the program several times, with different values entered for CCLKCFG, initially inthe range 2 to 9. For each record how many times the led flashes in 30 s.
1. Deduce what is the approximate duration of the delay function.
2. Which setting of CCLKCFG most closely matches your original reading?
Assignment Task 2: Switching off the main PLL
Compile and download Program Example 2 to an mbed. Carefully measure howmany times the LED flashes in one minute. It will be very slow. Can you explain thebeats per minute that you measure for this, comparing with the first value recorded in Assignment Task1?
Assignment Task 3: Adjusting the Phase Locked Loop
Adjust Program Example 2 to set a multiply value for the PLL. To do this, applythe code fragment of Program Example 3, inserting it before the while ( ) loop in Program Example 2. Basic information on setting PLL0CFG is given in Table 3.You can try your own experimental values. In each case, measure the LED blink rate, and try tocorrelate it to the setting you have made.
Assignment Task 4: Exploring mbed Power Consumption
Download Program Example 4 (or indeed any mbed program which does notrequire external connections) to an mbed. Disconnect the USB cable and power yourcircuit as shown in Fig. 5. Use a battery pack or bench DC supply, set at around6 V. The supply should link to the VIN mbed pin (pin 2), with an ammeterdforexample, a digital multimeter on its 200 mA rangedinserted between supply positiveand the VIN pin. The mbed will draw an approximately constant current, so theprecise supply voltage does not matter, only that it lies in the specified range of 4.5to 9 V.Now measure and record the current supplied to the mbed. You should find thissomewhere in the region of 140 mA.
Assignment Task 5: Switching Unwanted Things Off!
Create a new program using Program Example 15.4. Download to an mbed, and measure supply current as in Fig. 15.9. First comment out both the lines
PHY_PowerDown(); and
PCONP = 0x00008000;.
Run the program and measure current consumption. This should be the same as found in Assignment Task4. Enable each and then both of these lines, in each case recompiling the program, downloading, running, and reading the current consumption.
Record the values measured.
You will see a significant power reduction with the PHY switched off, and a lesser reduction with the peripherals all off. We are beginning to see that power consumption can be managed.
Assignment Task 6: Manipulating the Clock Frequency
We saw earlier in this chapter that the LPC1768 has extensive capabilities to vary the clock frequency. Now we understand one of the reasons why we can trade off speed of execution with power consumption. A program with significant computational demands will need to run fast, and will consume more power; one with low computational demands can run slowly, and consume less power.
- Rerun both Program Examples 15.1 and 15.2 in turn. Now measure current supply to the mbed for a range of different clock frequencies. Record your results.
- Write a program which both powers down unwanted peripheral devices, and slowsdown the clock. How low can you get the current consumption?
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