Dual Oscillator For Microcontrollers
Saturday, January 11, 2014
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The MAX7378 contains two oscillators and a power-on reset circuit for microprocessors. The Speed input selects either 32.768 kHz (LF) or a higher frequency, which is pre-programmed. The type number corresponds to the standard pre-programmed value and the value of the reset threshold. There is a choice of two threshold values: 2.56 V and 4.29 V.
Both thresholds are available with all standard frequencies, which are 1 MHz, 1.8432 MHz, 3.39545 MHz, 3.6864 MHz, 4 MHz, 4.1943 MHz, and 8 MHz. However, any frequency between 600 kHz and 10 MHz is also possible. An internal synchronization circuit ensures that no glitches occur when switching between the two oscillators. The Reset output of the MAX7378 is available in three different options.
Two of the options are push-pull types, either active low or active high. The third option is open drain, which thus requires an external pull-up resistor. That is the only standard option (which is why a resistor in dashed outline is shown connected to the Reset output). The Reset signal remains active for 100 µs after the supply voltage rises above the threshold voltage.
The Reset signal becomes active immediately if the voltage drops below the threshold. The IC is powered via two separate pins. The VL pin powers the reset and oscillator circuitry, while the VCC pin powers the remainder of the chip. The two pins must always have the same potential. Good decoupling in the form of two 100-nF ceramic capacitors (SMD types) is also necessary.
The IC is housed in the tiny 8-pin µMAX package and has dimensions of only 3.05 × 5.03 mm including pins, with a pin pitch of only 0.65 mm. Unfortunately, the accuracy of the oscillators is not especially good. The HF oscillator has an error of ±2% at 25 °C with a 5-V supply voltage and a maximum temperature coefficient of +325 ppm, which doesn’t exactly correspond to crystal accuracy, but it is certainly usable for most non-time-critical applications.
The error over the full supply voltage range (2.7–5.5 V) is twice as large. The 32.768-kHz oscillator is more accurate, with an error of only 1% at 5 V and 25 °C, although this is still a bit too much for time measurements. The error can be as much as ±3% over the entire supply voltage range. The maximum current consumption is 5.5 mA, which is relatively low.
Both thresholds are available with all standard frequencies, which are 1 MHz, 1.8432 MHz, 3.39545 MHz, 3.6864 MHz, 4 MHz, 4.1943 MHz, and 8 MHz. However, any frequency between 600 kHz and 10 MHz is also possible. An internal synchronization circuit ensures that no glitches occur when switching between the two oscillators. The Reset output of the MAX7378 is available in three different options.
Two of the options are push-pull types, either active low or active high. The third option is open drain, which thus requires an external pull-up resistor. That is the only standard option (which is why a resistor in dashed outline is shown connected to the Reset output). The Reset signal remains active for 100 µs after the supply voltage rises above the threshold voltage.
The Reset signal becomes active immediately if the voltage drops below the threshold. The IC is powered via two separate pins. The VL pin powers the reset and oscillator circuitry, while the VCC pin powers the remainder of the chip. The two pins must always have the same potential. Good decoupling in the form of two 100-nF ceramic capacitors (SMD types) is also necessary.
The IC is housed in the tiny 8-pin µMAX package and has dimensions of only 3.05 × 5.03 mm including pins, with a pin pitch of only 0.65 mm. Unfortunately, the accuracy of the oscillators is not especially good. The HF oscillator has an error of ±2% at 25 °C with a 5-V supply voltage and a maximum temperature coefficient of +325 ppm, which doesn’t exactly correspond to crystal accuracy, but it is certainly usable for most non-time-critical applications.
The error over the full supply voltage range (2.7–5.5 V) is twice as large. The 32.768-kHz oscillator is more accurate, with an error of only 1% at 5 V and 25 °C, although this is still a bit too much for time measurements. The error can be as much as ±3% over the entire supply voltage range. The maximum current consumption is 5.5 mA, which is relatively low.
As a result transistor T2 gets forward biased. This effectively grounds the base of transistor T1 which is thus cut off and the remaining circuit does not get any power supply. In this state, only a small (negligible) current is taken by the circuit, which will not affect the telephone line condition. However, when handset of any telephone connected to the telephone lines is lifted (off-hook), line voltage suddenly drops to about 10 volts. As a result, transistor T2 is switched off and transistor T1 gets forward biased via resistor R1. Now, the astable multivibrator built around timer IC1 starts oscillating and the speaker starts sounding.
For the diode radio (any germanium diode is suitable in this design) the key to success is correct impedance matching so that none of the received signal energy is lost. The antenna coil on the 10 mm diameter by 100 mm long ferrite rod is made up of 60 turns with a tap point at every 10 turns; this is suitable for medium wave reception. If a long external aerial is used it should be connected to a lower tap point to reduce its damping effect on the circuit. You can experiment with all the available tapping points to find the best reception. With such a simple radio design, the external aerial will have a big influence on its performance.
