NXP Semiconductors MPC5644A, Reference Manual, Page: 1633 / 1735

Periodic Interrupt Timer (PIT)
MPC5644A Microcontroller Reference Manual, Rev. 6
Freescale Semiconductor 1633
The PIT generates a real time interrupt when the selected interrupt time period elapses. The RTI interrupt
is disabled locally by setting the TIE bit to zero. The real time interrupt flag (TIF) is set to 1 when a timeout
occurs, and is cleared by writing a 1 to the TIF bit. (The flag will be set regardless of whether the interrupt
is enabled.)
The RTI can be used for periodic wakeup from a low power mode. It can also be used to generate a general
purpose interrupt.
34.6 Initialization and application information
34.6.1 Example configuration
In the example configuration
• the PIT clock has a frequency of 50 MHz
• the RTI clock has a frequency of 10 MHz
• the RTI shall be set up to create a wakeup interrupt every 500 ms
• timer 1 shall create an interrupt every 5.12 ms
• timer 3 shall create a trigger event every 30 ms
First the PIT module needs to be activated by writing a 0 to the MDIS bit in the PITMCR.
The 50 MHz clock frequency equates to a clock period of 20 ns and the 10 MHz frequency equates to a
clock period of 100 ns. Therefore the RTI timer needs to trigger every 500 ms/100 ns = 5000000 cycles.
Timer 1 needs to trigger every 5.12 ms/20 ns = 256000 cycles and timer 3 every 30 ms/20 ns = 1500000
cycles. The value for the LDVAL register trigger would be calculated as (period / clock period)

1.
This means that RTI LDVAL will be written with 0x004C_4B3F, LDVAL1 with 0x0003_E7FF and
LDVAL3 with 0x0016_E35F.
To generate the wakeup interrupt, the interrupt line must be enabled by writing a 1 to the RTI TIE bit in
the TCTRL register. To start the RTI, the TEN bit in the RTI TCTRL register must also be set.
The interrupt for Timer 1 is enabled by setting the TIE bit in the TCTRL1 register. The timer is started by
writing a 1 to bit TEN in the TCTRL1 register.
Timer 3 shall be used only for triggering. Therefore Timer 3 is started by writing a 1 to bit TEN in the
TCTRL3 register, bit TIE stays at 0.
The following example code matches the described setup:
// turn on PIT
PIT_CTRL = 0x00;
// RTI
PIT_RTI_LDVAL = 0x004C4B3F; // setup RTI for 5000000 cycles
PIT_RTI_TCTRL = PIT_TIE; // let RTI generate interrupts
PIT_RTI_TCTRL |= PIT_TEN; // start RTI
// Timer 1
PIT_LDVAL1 = 0x0003E7FF; // setup timer 1 for 256000 cycles
PIT_TCTRL1 = TIE; // enable Timer 1 interrupts