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The MSP430 device’s timer system is broken up into 2 primary parts: the general purpose timers, like TIMER_A and TIMER_B, and the interval Watchdog timer. SYS/BIOS requires something to drive its time base if the application needs to use any SYS/BIOS time-based services, for example, a kernel object that time out after being blocked for some duration. In practice, that means that SYS/BIOS can either utilize one of the device’s Timer_A or Timer_B modules, most MSP430s have a mix of several of these modules. SYS/BIOS assigns a logical ID to each of these modules, with Timer_A1 assigned a logical ID of zero, Timer_A2 assigned a logical ID of one, until all the Timer_A modules are numbered, then Timer_B1 takes the next logical ID and so on. These logical IDs become important if the developer wants to configure SYS/BIOS to use a specific timer module. Shown here one can see a screen shot of the SYS/BIOS configure tool’s Timer options and the developer can see that one of the fields that can be specified is the logical ID of the timer device. If the developer fills in a value of zero or greater, then one is selecting a specific Timer module to use via its logical ID. If one leaves the value as the default value of -1,then the developer is telling SYS/BIOS that it can select any timer module that is not already allocated for use by SYS/BIOS.
