В доке написано , что countdown, но тем не менее alt_timestamp дает увеличивающиеся значения во времени

Nios II Software Developer’s Handbook (6-18):
"Sometimes you want to measure time intervals with a degree of accuracy greater than
that provided by HAL system clock ticks. The HAL provides high resolution timing
functions using a timestamp driver. A timestamp driver provides a monotonically
increasing counter that you can sample to obtain timing information. The HAL only
supports one timestamp driver in the system."

ясно, спасибо