13.3.1
GPTM Reset Conditions
After reset has been applied to the GPTM module, the module is in an inactive state, and all control
registers are cleared and in their default states. Counters Timer A and Timer B are initialized to all
1s, along with their corresponding registers:
■ Load Registers:
–
GPTM Timer A Interval Load (GPTMTAILR)
register (see page 1004)
–
GPTM Timer B Interval Load (GPTMTBILR)
register (see page 1005)
■ Shadow Registers:
–
GPTM Timer A Value (GPTMTAV)
register (see page 1014)
–
GPTM Timer B Value (GPTMTBV)
register (see page 1015)
The following prescale counters are initialized to all 0s:
■
GPTM Timer A Prescale (GPTMTAPR)
register (see page 1008)
■
GPTM Timer B Prescale (GPTMTBPR)
register (see page 1009)
■
GPTM Timer A Prescale Snapshot (GPTMTAPS)
register (see page 1017)
■
GPTM Timer B Prescale Snapshot (GPTMTBPS)
register (see page 1018)
13.3.2
Timer Clock Source
The general purpose timer has the capability of being clocked by either the system clock or an
alternate clock source. By setting the
ALTCLK
bit in the
GPTM Clock Configuration (GPTMCC)
register, offset 0xFC8, software can selects an alternate clock source as programmed in the
Alternate
Clock Configuration (ALTCLKCFG)
register, offset 0x138 in the System Control Module. The
alternate clock source options available are PIOSC, RTCOSC and LFIOSC. Refer to “System
Control” on page 220 for additional information.
Note:
When the
ALTCLK
bit is set in the
GPTMCC
register to enable using the alternate clock
source, the synchronization imposes restrictions on the starting count value (down-count),
terminal value (up-count) and the match value. This restriction applies to all modes of
operation. Each event must be spaced by 4 Timer (ALTCLK) clock p 2 system clock
periods. If some events do not meet this requirement, then it is possible that the timer block
may need to be reset for correct functionality to be restored.
Example: ALTCLK= T
PIOSC
= 62.5ns (16Mhz Trimmed)
T
hclk
= 1us (1Mhz)
4*62.5ns + 2*1us = 2.25us 2.25us/62.5ns = 36 or 0x23
The minimum values for the periodic or one-shot with a match interrupt enabled are:
GPTMTAMATCHR
= 0x23
GPTMTAILR
= 0x46
13.3.3
Timer Modes
This section describes the operation of the various timer modes. When using Timer A and Timer B
in concatenated mode, only the Timer A control and status bits must be used; there is no need to
use Timer B control and status bits. The GPTM is placed into individual/split mode by writing a value
959
June 18, 2014
Texas Instruments-Production Data
Tiva
™
TM4C1294NCPDT Microcontroller