Introduction to Microcontrollers Part 2
“Generator” – Power Circuitry
In our last session we looked at the basic structure and operation of a microcontroller (MCU). Now let’s look at some of the hardware (peripheral circuitry) required to support the microprocessor. In particular, we will look at some hardware used in the Renesas RL78 Family (RL78/G13), one of the new-generation general-purpose MCUs.
An MCU, like any of its various components introduced in Digital Circuits, needs a power supply to drive it. So it must be connected to an outside battery or other suitable power source. Figure 1 shows the pin arrangement on a 20-pin RL78 Family (RL78/G13) chip. Pin 9 (VSS) and pin 10 (VDD) are the power pins, which connect as follows:
- Pin 9 (VSS) to GND.
- Pin 10 (VDD) to the positive terminal of the power supply.
The datasheet (hardware manual) for the RL78 Family (RL78/G13) indicates that the power voltage (VDD) must be between 1.6 and 5.5 V. This means that the MCU is guaranteed to run when supplied with any voltage within this range. This voltage range is referred to as the operating voltage, or, in some hardware manuals, as the recommended operating voltage.
Figure 2 shows an example of an actual power-connection configuration of a 20-pin RL78 Family (RL78/G13) MCU.
Pin 10 connects to bypass capacitor C1. This bypass prevents malfunctions that might otherwise occur when a current spike causes the voltage to drop. A typical bypass capacitor is a ceramic capacitor with capacitance between 0.01 and 0.1 µF.
The power-supply voltage is stepped down by an internal regulator to the voltage used to drive the MCU’s internal circuitry: that is, to either 1.8 V or 2.1 V. The regulator itself is stabilized by another capacitor, C2, at pin 8.
“Conductor” – Oscillators
As we saw in our third session on digital circuitry basics, sequential circuits operate in sync with the rising or falling edge of a clock (CK) signal. MCUs consist of sequential circuits, and so they require a CK signal. This external clock signal is provided by an external oscillator connected to the MCU.
Figure 3 shows an example of an external oscillator connected to an RL78 Family (RL78/G13) MCU. Specifically, a crystal oscillator is connected to pins X1 and X2. The MCU includes two internal clock oscillators that work in conjunction with the external clock signal.
- The main clock drives the CPU.
- The sub-clock is typically used with peripheral circuits or as a real-time clock.
Because the RL78 Family (RL78/G13) uses a highly precise on-chip oscillator (accurate to within 1%) to drive its robust set of peripheral circuitry, it can operate without need of an external clock. MCUs driven by internal clocks are less expensive to design.
Even where an on-chip oscillator is present, however, an external crystal oscillator may be used in cases where it is necessary to achieve better precision and lower temperature-induced variation; for example, in MCUs used to control watches and so on.
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