LM3S308 Luminary Micro, Inc, LM3S308 Datasheet - Page 166

• Manufacturer Part Number: LM3S308
• Description: Lm3s308 Arm Microcontroller
• Manufacturer: Luminary Micro, Inc

General-Purpose Timers
9.2.2
9.2.2.1
166
(GPTMTAILR) register (see page 190) and the GPTM TimerB Interval Load (GPTMTBILR) register
(see page 191). The prescale counters are initialized to 0x00: the GPTM TimerA Prescale
(GPTMTAPR) register (see page 194) and the GPTM TimerB Prescale (GPTMTBPR) register (see
page 195).
32-Bit Timer Operating Modes
This section describes the three GPTM 32-bit timer modes (One-Shot, Periodic, and RTC) and their
configuration.
The GPTM is placed into 32-bit mode by writing a 0 (One-Shot/Periodic 32-bit timer mode) or a 1
(RTC mode) to the GPTM Configuration (GPTMCFG) register. In both configurations, certain GPTM
registers are concatenated to form pseudo 32-bit registers. These registers include:
■
■
■
■
In the 32-bit modes, the GPTM translates a 32-bit write access to GPTMTAILR into a write access
to both GPTMTAILR and GPTMTBILR. The resulting word ordering for such a write operation is:
GPTMTBILR[15:0]:GPTMTAILR[15:0]
Likewise, a read access to GPTMTAR returns the value:
GPTMTBR[15:0]:GPTMTAR[15:0]
32-Bit One-Shot/Periodic Timer Mode
In 32-bit one-shot and periodic timer modes, the concatenated versions of the TimerA and TimerB
registers are configured as a 32-bit down-counter. The selection of one-shot or periodic mode is
determined by the value written to the TAMR field of the GPTM TimerA Mode (GPTMTAMR) register
(see page 177), and there is no need to write to the GPTM TimerB Mode (GPTMTBMR) register.
When software writes the TAEN bit in the GPTM Control (GPTMCTL) register (see page 181), the
timer begins counting down from its preloaded value. Once the 0x0000.0000 state is reached, the
timer reloads its start value from the concatenated GPTMTAILR on the next cycle. If configured to
be a one-shot timer, the timer stops counting and clears the TAEN bit in the GPTMCTL register. If
configured as a periodic timer, it continues counting.
In addition to reloading the count value, the GPTM generates interrupts and triggers when it reaches
the 0x000.0000 state. The GPTM sets the TATORIS bit in the GPTM Raw Interrupt Status
(GPTMRIS) register (see page 186), and holds it until it is cleared by writing the GPTM Interrupt
Clear (GPTMICR) register (see page 188). If the time-out interrupt is enabled in the GPTM Interrupt
Mask (GPTIMR) register (see page 184), the GPTM also sets the TATOMIS bit in the GPTM Masked
Interrupt Status (GPTMMIS) register (see page 187). The trigger is enabled by setting the TAOTE
bit in GPTMCTL, and can trigger SoC-level events such as ADC conversions.
If software reloads the GPTMTAILR register while the counter is running, the counter loads the new
value on the next clock cycle and continues counting from the new value.
If the TASTALL bit in the GPTMCTL register is asserted, the timer freezes counting until the signal
is deasserted.
GPTM TimerA Interval Load (GPTMTAILR) register [15:0], see page 190
GPTM TimerB Interval Load (GPTMTBILR) register [15:0], see page 191
GPTM TimerA (GPTMTAR) register [15:0], see page 198
GPTM TimerB (GPTMTBR) register [15:0], see page 199
Preliminary
June 04, 2008