121. Mandatory Build-Time Timebase Selection

You are implementing firmware for a system that depends on a hardware timebase.

Exactly one timebase must be selected at build time.
Runtime selection is architecturally forbidden.

Your task is to complete the program so that:

• The timebase is selected only at compile time
• The program cannot build unless a valid timebase type is chosen
• No runtime configuration, branching, or fallback is possible
• This must be enforced using the C++ type system, not comments or conventions

Input / Program Flow:

The firmware performs the following steps:

1. Reads a tick counter from a hardware timebase
2. Converts ticks to milliseconds
3. Prints the elapsed time

Two valid timebases exist:

• SysTickTimer
  • Frequency: 1 kHz
  • Tick count: 100
• RtcTimer
  • Frequency: 32 Hz
  • Tick count: 4

Only one timebase is ever used in a firmware build.

Output:

The program must print exactly:

elapsed_ms=100 

Note:
This output corresponds to selecting SysTickTimer.
Validation of RtcTimer is performed by recompiling the program with a different compile-time selection.

Constraints:

• No virtual functions
• No inheritance used for polymorphism
• No runtime conditionals (if, switch)
• No heap allocation
• No STL containers
• No macros for selection
• Timebase selection must be enforced at compile time
• Program must fail to build if selection is missing