Embedded Systems Design with the Atmel AVR Microcontroller Part II

236 CHAPTER 8. SYSTEM LEVEL DESIGN
8.2 WEATHER STATION
In this project, we design a weather station to sense wind direction and to measure ambient temperature.
The measure temperature values are displayed on an LCD in Fahrenheit. The wind direction
is displayed on LEDs arranged in a circular pattern. The wind direction and temperature are transmitted
serially to an external device.
8.2.1 REQUIREMENTS
The requirements for this system include:
Sense wind direction and measure ambient temperature.
Display on an LCD.
Display measured temperature in Fahrenheit on an LCD.
Display wind direction on LEDs arranged in a circular pattern.
Transmit serially wind direction and temperature data.
8.2.2 STRUCTURE CHART
To begin the design process,a structure chart is used to partition the system into definable subsystems.
We employ a top-down design/bottom-up implementation approach. The structure chart for the
weather station is shown in Figure 8.1. The three main microcontroller subsystems needed for this
project are the USART for serial communication, the ADC system to convert the analog voltages
from the LM34 temperature sensor and a weather vane into digital signals, and the wind direction
display. This display consists of a 74154, 4-to-16 decoder and 16 individual LEDs to display wind
direction. The system is partitioned until the lowest level of the structure chart contains “doable”
pieces of hardware components or software functions. Data flow is shown on the structure chart as
directed arrows.
8.2.3 CIRCUIT DIAGRAM
The circuit diagram for the weather station is shown in Figure 8.2. The weather station is equipped
with two input sensors: the LM34 to measure temperature and the weather vane to measure wind
direction. Both of the sensors provide an analog output that is fed to PORTA[0] and PORTA[1].
The LM34 provides 10 mV output per degree Fahrenheit. The weather vane provides 0 to 5 VDC
for 360 degrees of vane rotation. The weather vane must be oriented to a known direction with the
output voltage at this direction noted. We assume that 0 VDC corresponds to North and the voltage
increases as the vane rotates clockwise to the East. The vane output voltage continues to increase
until North is again reached at 5 VDC and then rolls over back to zero volts. All other directions
are derived from this reference point.