Reading And Programming Timers

The core operation is divisor programming.

divisor = base_frequency / requested_frequency

After computing the divisor, write a control word, then the divisor low byte, then the divisor high byte. This is a good place to slow down and make the relationship between arithmetic and protocol visible. The arithmetic decides what value the controller should use. The protocol decides how that value is delivered to the controller.

Validation

Reject impossible requests before touching the device. An invalid timer number, a requested frequency of zero, a divisor outside the 16-bit range, or a null output pointer for a status read should fail before any controller state is changed. This is not just defensive programming; it makes tests deterministic. If a function fails halfway through device configuration, later tests may begin from a surprising state.

Status Reads

Status reads are useful for tests and debugging. They prove that your code can ask the controller what it is currently configured to do, not only write new configuration.

They also train an important habit. Many devices are not write-only command sinks. Good low-level code can inspect status, explain the current state, and use that information to recover from errors.

Practical Debugging

When stuck, print the requested frequency, computed divisor, low byte, high byte, and control word in hexadecimal. These values are small enough to verify by hand. If the output does not match your expectation, the bug is usually in field construction, integer division, or byte order.