Texas Instruments MSP50C614 manual Timer Registers

Timer Registers

Selection between the timer-source options is made using two control bits in the interrupt/general control register (IntGenCtrl). The IntGenCtrl is a 16-bit port-addressed register at 0x38. Clearing bit 8 selects 1/2 MC as the source for TIMER1. Setting bit 8 selects the reference oscillator as the source for TIM- ER1. Similarly, clearing bit 9 of the IntGenCtrl selects 1/2 MC as the source for TIMER2. Setting bit 9 selects the reference oscillator as the source for TIM- ER2. The default value after a RESET LOW is zero: select 1/2 MC as the source.

Each of the TIMERs counts from the value stored in its period register to 0x0000. These maximum and minimum counts each receive a full clock cycle from the TIMER source. This means that the true period of the TIMER, from one underflow event to the next, is the value stored in the period register plus one:

Time duration btwn. underflows = (value in PRD + 1) ⎟(frequency of Timer

Source)

TIMER1 and TIMER2 must be enabled for use. This is done at the IntGenCtrl register. Bit 10 of the IntGenCtrl is the enable bit for TIMER1, and bit 11 is the enable bit for TIMER2. Setting the enable bit enables the TIMER, i.e., starts count-down running. Clearing the enable bit disables the TIMER, i.e., stops the count-down. The default setting after a RESET LOW is zero: both TIMERs disabled. Refer to Section 3.4, Interrupt/General Control Register, for sum- mary information regarding the IntGenCtrl.

The TIMER enable bits may be used to start and stop the TIMERs repeatedly in software. Switching the enable bit from 1 to 0 stops the TIMER, but the current value in the count-down register is retained. When the enable bit is subsequently switched from 0 to 1, count-down then resumes from the held value. The following procedure outlines one (of many) possible ways to start the TIMERs. TIMER2 is given as an example:

1)Select the TIMER2 clock source: 1/2 MC or RTO/CRO (bit 9 of the Int- GenCtrl, address 0x38).

2)Clear the TIMER2 enable (bit 11 in the IntGenCtrl).

3)Load the count-down register (TIM2) with the desired period value ahead- of-time. This prepares TIM2 for counting, and also loads the period regis- ter (PRD2) with its value.

4)Be sure the TIMER2 interrupt (INT2) has been enabled for service (set bit 2 of IntGenCtrl).

5)Flip the TIMER2 enable bit from 0 to 1, at the precise time you want count- ing to begin.

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