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Instead of

_delay_ms(100);

could just

set_sleep_mode(SLEEP_MODE_PWR_DOWN);
sleep_mode();

and get woken up by watchdog after 1 second. The sensors certainly don't mind to have 1s instead of 100ms to settle.

Power consumption should be 1s * ~250uA vs. 100ms * ~6mA.

To be tested & measured.

reduces wear of display

• MCP1501-30E/SN
• use also for TMP36
• do not set AVCC as reference voltage in initADC()

Currently, the framebuffer in SRAM is set/cleared in byte mode. Sequential mode should be more efficient.

• use watchdog or timer2
• consider to use external 32kHz crystal for timer2
• measure only every 20 or 30 seconds?
• update display only every 5 minutes?

detect low battery (BOD?) and display a warning

Makefile

• Option -D__AVR_ATmega328P__ should be removed. This is a built-in define, and provided by specs-atmega328p since v5. If the compiler does not work as expected without defining that macro by hand, the toolchain is broken.
• Option -Wl,--relax bypasses the assembler and should be replaced by -mrelax. Otherwise, the assembler might compute symbol values / offsets at assembly time, but which are actually not computable at assembly time because linker relaxation might change their values.
• thermidity.elf depends on header files like font.h, which is not described in the current Makefile. This means that when you only change some header(s) and then rerun make, it won't re-build thermidity.elf.

In functions like sram::sramWriteString, display::writeString and uart::printString that get a (pointer-to) string argument, the pointed-to object should be const because these functions are not supposed to change the pointed-to object.

For example, usart::printString should have prototype

void printString (const char *data) (or void printString (const char *const data) if you wish).

Notice that it makes a difference on which side of the * the const is placed:

• Left of the *, the const refers to the pointed-to object.
• Right of the *, the const refers to the pointer itself, i.e. to data in the example.

In most cases, it is fine to change pointers like data, e.g. by using them to iterate over the chars in a string, whereas for output functions like above, they should still work as expected and without diagnostic in contexts like:

const char text[] = "Hallo";
printString (text);

As a aside, this applies to other qualifiers like volatile or __flash as well, and also to pointers of higher order:

In volatile int *p;, p points to a volatile object but b itself is not volatile, whereas in int * volatile p;, p itself is volatile but the pointed-to object is not, and in volatile int * volatile p;, both p and the pointed-to object are volatile.

In int **p; there are 8 = 2³ possible ways to put volatile's.

• 1.5s 3mA vs. 3s 3mA
• sufficient to update at all?
• recommendation is to make a full update after 5 fast/partial updates

• humidity sensor is ratiometric
• use a thermistor instead of TMP36
• no need for a reference voltage; use ATmega's internal 1.1V reference to measure battery voltage
• neither humidity sensor nor thermistor (?) require a bypass capacitor so MCU output pin can easily switch their supply current