Example chapter04_04 uses an intuitive LED class hierarchy to exemplify object oriented design and polymorphism. In this example, five LEDs in total having three individual kinds of port control are toggled in near unison with a frequency of approximately 1/2Hz.
The class hierarchy consists of an LED base class called pwm_base
from which two LED classes are derived, led_port and led_pwm.
The port class led_port itself uses two kinds of ports,
one microcontroller port and three other ports on an external
serial SPI(TM) port axpander chip of type MICROCHIP(R) MCP23S17.
The PWM based LED is toggled via the dimming of the PWM duty cycle.
The port expander chip is controlled with SPI(TM). In this particular example, an all-software bus has been developed using four microcontroller port pins toggled manually. This SPI(TM) communication class itself is derived from a communication base class that provides a uniform interface for rudimentary byte read and write functions.
The application places five LED base class pointers in an
std::array and subsequently toggles them in a range-based
for-loop in the application task.
The LED base class pointers, for instance, are shown below.
std::array<mcal::led::led_base*, 5U> app_led_base_class_pointers =
{{
mcal::led::led0(),
mcal::led::led1(),
mcal::led::led2(),
mcal::led::led3(),
mcal::led::led4()
}};
The application task performs the toggle functionality at a frequency of approximately 1/2Hz using dynamic polymorphism on the base class pointer list.
void app::led::task_func()
{
if(app_led_timer.timeout())
{
app_led_timer.start_interval(app_led_one_sec);
for(auto& p_led : app_led_base_class_pointers)
{
p_led->toggle();
}
app_led_state_is_on = (!app_led_state_is_on);
}
}
The five discrete LEDs in this example are fitted and setup in the following way:
| LED | Port | Details |
|---|---|---|
| LED0 | microcontroller portb.5 |
port toggle high / low, 750 Ohm |
| LED1 | microcontroller portb.1 |
Timer A, PWM duty cycle 100% / 0%, 750 Ohm |
| LED2 | port expander pin GPA0 |
port toggle high / low, SPI(TM) software drive, 750 Ohm |
| LED3 | port expander pin GPA1 |
port toggle high / low, SPI(TM) software drive, 750 Ohm |
| LED4 | port expander pin GPA2 |
port toggle high / low, SPI(TM) software drive, 750 Ohm |
In this example, we use ports from both the microcontroller as well
as an external port expander chip. Hardware adressing is used
on the port expander chip. The port expander address is
hard-wired to the value 7 via connecting each of the three
pins A0 ... A2 to +5V.
The connections of the port expander chip are tabulated below.
| Port Expander Pin | Expander Function | Connection |
|---|---|---|
| 9 | VDD | +5V |
| 10 | VSS | GND |
| 11 | CS (NOT) | portc.4 |
| 12 | SCK | portc.3 |
| 13 | SI | portc.1 |
| 14 | SO | portc.2 |
| 15 | A0 | +5V |
| 16 | A1 | +5V |
| 17 | A2 | +5V |
| 18 | RESET (NOT) | 15 kOhm to +5V pullup over switch to GND |
| 21 | GPA0 | 750 Ohm to LED2 |
| 22 | GPA1 | 750 Ohm to LED3 |
| 23 | GPA2 | 750 Ohm to LED4 |
The hardware setup is pictured in the image below.
