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FIGURE 5.6 Ceramic resonator connected to the oscillator pins of the AVR processor.
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FIGURE 5.7 A quartz crystal connection to the oscillator pins of the AVR processor.
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90 AVR HARDWARE DESIGN ISSUES
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A regular parallel resonant crystal start-up time
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Voltage input switched ON here
FIGURE 5.8 Oscillator start-up using a parallel resonant crystal after the supply input is applied to the processor.
USING A QUARTZ CLOCK CRYSTAL
For low-power operation, a general rule is to keep the operating frequency down. Typical operating frequency versus current graphs provided by the manufacturer substantiate this. However, such graphs supplied by the manufacturer usually plot frequency starting at about 1 MHz going up to 10 20 MHz or so. It seems that extrapolating the curve to lower frequency would result in a further-reduced operating current. Lower frequency crystals, typically 32 kHz, are very commonly available for use in calendar clock circuits. It seems logical that using a 32-kHz crystal to generate the operating clock for an AVR circuit would reduce the operating current substantially. I used the circuit illustrated in Figure 5.9 to test out this hypothesis. Figure 5.10 illustrates the start-up time for the 32-kHz crystal, which is about 2s. The oscillator showed start-up problems and I had to play with the C1, C2, and R1 values to get the circuit to oscillate. Table 5.4 illustrates my findings. However, more importantly, the current consumption is not reduced at all, as illustrated in Table 5.4. Figure 5.11 illustrates an oscilloscope screen shot of the oscillations and the dynamic current consumption by the processor. The current consumption peaks during the times when the oscillations are in a transition phase. It is concluded that the 32-kHz calendar clock crystal is not a viable component to be used with the AVR processor in terms of current savings.
USING INTERNAL RC CLOCK OSCILLATOR
The last option for clocking the AVR processor is to use the internal RC oscillator available on some of the processors (AT90S1200, 2343, Tiny22). The AT90S1200 is shipped with the internal RC oscillator disabled and can be reprogrammed with the help of a parallel programmer to enable the RCEN bit so as to select the RC oscillator. However, AT90S1200A can be used, which has the RC oscillator enabled.
OPERATING CLOCK SOURCES 91
FIGURE 5.9 Circuit schematic for the 32-kHz clock crystal test circuit.
32KHz series resonant crystal start-up. Input Voltage is applied here.
Oscillations stabilize here
Program to pulse the PORTB bits starts executing here
FIGURE 5.10 32-kHz oscillator start-up time.
The AT90S2343 and the Tiny22 are shipped with the RCEN bit enabled (i.e., at 0 ) and the internal RC oscillator can be used right away. The RC oscillator has supply voltage dependence, and consequently this clocking option should only be used if the application does not require timing accuracy. I determined the frequency variation as a function of supply voltage with the help of a circuit illustrated in Figure 5.12. The supply voltage was varied between 2 V and 5.9 V and the corresponding pulse output was measured. The pulse output was the result of the program running on the 2343 processor, and the pulse frequency was related to the clock frequency as four clock cycles generating one pulse output cycle. By measuring the pulse frequency, the clock frequency was deduced and plotted, as illustrated in Figure 5.13 as well as in Table 5.5.
92 AVR HARDWARE DESIGN ISSUES
TABLE 5-4 32-KHZ OSCILLATOR START-UP TIMES AND CURRENT CONSUMPTION FOR VARIOUS CAPACITOR AND RESISTOR VALUES R1 C1 C2 START-UP TIME (SECONDS) I (DC) MA
82 K 82 K 82 K 120 K 120 K 120 K 120 K 390 K 390 K 390 K 470 K 470 K 470 K 470 K 910 K 910 K 910 K
33 pF 68 pF 68 pF 68 pF 68 pF 33 pF 68 pF 68 pF 33 pF 33 pF 33 pF 68 pF 68 pF 68 pF 68 pF
68 pF 68 pF 33 pF 33 pF 33 pF 68 pF 68 pF
Doesn t oscillate 8-10 8-10 3-5 3-5 Doesn t oscillate 10 3-4 3 3 2-3 2-3 3-4 3 2-3 Doesn t oscillate 4-5
5.83 5.5 5 5 5.7 4 4 4 3.9 3.9 4 4 4.4 4.4
Clock signal at X2 pin with a series resonant 32kHz crystal on an AT90S1200
4 mA/div. Current consumption by the AT90S1200 without any external load
FIGURE 5.11 Current consumption by an AT90S1200 processor when operated with a 32-kHz clock crystal.
RESET CIRCUIT 93
Variable Supply
Vcc PB0 To Frequency Counter
FIGURE 5.12 Circuit to measure the oscillator frequency variation as a function of supply voltage.
1600 1400 Oscillator Frequency (kHz) 1200 1000 800 600 400 200 0 2 2.5 3 3.5 4 4.5 Supply Voltage (V) 5 5.5 6 6.5
FIGURE 5.13 Variation of RC system clock frequency as a function of supply voltage.
The program rc_calib.asm in the code directory was used to measure the frequency of the output waveform generated on the PORTB pins. This frequency was then used to calculate the internal clock frequency of the processor.
5.3 Reset Circuit
CPUs require a reset pulse after the power supply has stabilized. The basic requirement is that a processor reset pulse should appear after the power supply has settled to a stable value. This is to initialize the internal registers and the control circuit. Usually, the processor has a power-on reset circuit as well as an external reset input circuit. The power-on reset circuit activates when the power supply voltage is below a certain threshold. After some
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