DS3231 RTC with 7-segment 8 digits display using MAX7219
The ATmega is interrupted every second to read the time from a RTC DS3231 using I2C and to write to a MAX7219 display using SPI. MAX7219 code B decode mode is used to match the DS3231, every RTC byte represents two BCD digits.
During setup and once every minute the temperature is updated.
The temperature is represented by two bytes: ((temp_msb << 8) | temp_lsb) / 256.0.
One can select the temperature to round up, display the real part as a dot, or truncate using the TEMP flag.
The temperature is expected to stay between 0 and 68 degrees Celsius, which matches the DS3231S operating range, but not the DS3231SN.
This limitation is only to conform with the fast division by 10 without hardware division: (temp_msb * 26) >> 8, which only takes one multiply instruction while discarding the LSB.
Note that there are no delays in the code, as the TWI 400KHz for the DS3231 I2C waits a register change, and the MAX7219 can operate at 10MHz, faster than any 16MHz ATmega hardware SPI configuration. The FAST flag selects between bare metal and Arduino code, with the bare metal version using close to 500 bytes of Flash. The SPI_HARD flag can be enabled to make use of the faster SPI hardware, but requires specific pins. Software SPI is useful to keep all pins on the same side of an Arduino Nano.
- ATmega8/48/88/168/328 compatible board
- 7-segment 8 digits MAX7219 module
- RTC DS3231 module
| Pin | Module |
|---|---|
| 5V | VCC MAX7219 |
| A0 (PC0) or D11 (PB3) | SPI MOSI |
| A1 (PC1) or D10 (PB2) | SPI CS |
| A2 (PC2) or D13 (PB5) | SPI CLK |
| 3.3V | VCC DS3231 |
| A4 (PC4) | I2C SDA |
| A5 (PC5) | I2C SCL |
| GND | GND |