Embedded Systems Design with the Atmel AVR Microcontroller Part II

216 CHAPTER 7. ATMEL AVR OPERATING PARAMETERS AND INTERFACING
{
if(column==64)
{
lcd_select_side(RIGHT);
lcd_instruction_write(X_ADRESS | page);
lcd_instruction_write(Y_ADRESS);
}
lcd_data_write(0x00);
}
}
}
//*************************************************************************
7.4.8 HIGH POWER DC DEVICES
A number of direct current devices may be controlled with an electronic switching device such as
a MOSFET. Specifically, an N-channel enhancement MOSFET (metal oxide semiconductor field
effect transistor) may be used to switch a high current load on and off (such as a motor), using
a low current control signal from a microcontroller as shown in Figure 7.17(a). The low current
control signal from the microcontroller is connected to the gate of the MOSFET. The MOSFET
switches the high current load on and off, consistent with the control signal. The high current load
is connected between the load supply and the MOSFET drain. It is important to note that the load
supply voltage and the microcontroller supply voltage do not have to be at the same value. When
the control signal on the MOSFET gate is logic high, the load current flows from drain to source.
When the control signal applied to the gate is logic low, no load current flows. Thus, the high power
load is turned on and off by the low power control signal from the microcontroller.
Often the MOSFET is used to control a high power motor load. A motor is a notorious source
of noise. To isolate the microcontroller from the motor noise, an optical isolator may be used as an
interface as shown in Figure 7.17(b). The link between the control signal from the microcontroller
to the high power load is via an optical link contained within a Solid State Relay (SSR). The SSR
is properly biased using techniques previously discussed.
7.5 DC SOLENOID CONTROL
The interface circuit for a DC solenoid is provided in Figure 7.18. A solenoid provides a mechanical
insertion (or extraction) when asserted. In the interface, an optical isolator is used between the
microcontroller and the MOSFET used to activate the solenoid. A reverse biased diode is placed