Difference in MSEC_TO_UNITS parameters

[Artucas]

Hello, this might be a silly question, but i've seen various Nordic examples and i don't know whats the difference?

#define MIN_CONN_INTERVAL               MSEC_TO_UNITS(500, UNIT_1_25_MS)           /**< Minimum  acceptable connection interval (0.5 seconds). */
#define CONN_SUP_TIMEOUT                MSEC_TO_UNITS(4000, UNIT_10_MS)            /**< Connection supervisory timeout (4 seconds). */
#define NON_CONNECTABLE_ADV_INTERVAL    MSEC_TO_UNITS(100, UNIT_0_625_MS) /**< The advertising interval for non-connectable advertisement (100 ms). */

In all these examples above, first parameter to the macro is number of milliseconds, and the second parameter is the resolution. By changing the resolution, we change the number of ticks. But what is the difference? Which part of the software or hardware ar using that number of ticks? So my question comes down to: what is the difference between these statements, which one is better and will in all three cases MIN_CONN_INTERVAL will be exactly 500ms?

#define MIN_CONN_INTERVAL1    MSEC_TO_UNITS(500, UNIT_0_625_MS)
#define MIN_CONN_INTERVAL2    MSEC_TO_UNITS(500, UNIT_1_25_MS)
#define MIN_CONN_INTERVAL3   MSEC_TO_UNITS(500, UNIT_10_MS)

[Paolo D]

Those units are defined in the Bluetooth protocol. The connection interval has to be in units of 1.25ms, and the connection supervision timeout has to be in units of 10ms. So only MSEC_TO_UNITS(500, UNIT_1_25_MS) would work correctly for setting the connection interval.

[Paolo D]

That's not going to work, the connection supervision timeout needs to be longer than the minimum connection interval, at least a few seconds. The connection interval defines how often the devices communicate. Every time they communicate successfully, the connection supervision timer is reset. If the connection supervision timer reaches the timeout without a successful connection event occurring, the connection is lost. So the connection supervision timeout needs to be long enough that a few connection events can fail without causing the connection to be lost.