Schematic | Circuit guide | Manual Wiring diagram | Electronic

Dual Oscillator For Microcontrollers

Posted by merzaha gulhku Saturday, January 11, 2014 0 comments
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.