SCHEMATICS
Based on the component selection made earlier, we can now look at the detailed schematic diagram of the application
(see Figure).
The LCD and the sensor are connected to GPIO pins of the microcontroller. The LCD drive waveforms and SPI communication interface for the sensor are created in software.
The CPU is clocked by an external oscillator. To further reduce the overall system cost, the oscillator can be replaced by a low–cost resonator since accurate timing is not required or the CPU can be replaced by MC68HC908JL3 (as mentioned above).
The push button is connected to the IRQ pin of the CPU. Button depression can wake–up the CPU from low power stop mode even when the oscillator is stopped to minimize the power consumption.
During SW debugging, the code of the application is downloaded to the CPU through the SCI port (RS–232 protocol) by utilizing the serial bootloader. This path can also be used for future firmware upgrades.
The bill of materials is shown under Table.
| Item | Quantity | Reference | Part |
| 1 | 1 | BT1 | CR2032, 3 V Lithium |
| 2 | 2 | C2, C1 | 22n |
| 3 | 1 | C3 | 470n |
| 4 | 2 | C4, C5 | 100n |
| 5 | 1 | D1 | LCD, 2 x 11 Segments |
| 6 | 1 | J2 | Connectors and MPXY8010 |
| 7 | 1 | J3 | Header for Debugging |
| 8 | 1 | R1 | 330k |
| 9 | 1 | R2 | 10M |
| 10 | 4 | R3, R4, R5, R6 | 10k |
| 11 | 1 | SW1 | Push Button |
| 12 | 1 | U1 | MC68HC908GR8 |
| 13 | 1 | Y1 | Crystal, 32.768 kHz |