TC1784
System Timer (STM)
User´s Manual
14-1
V1.1, 2011-05
STM, V1.41
14
System Timer (STM)
This chapter describes the System Timer (STM). The TC1784’s STM is designed for
global system timing applications requiring both high precision and long period.
14.1
Overview
The STM has the following features:
•
Free-running 56-bit counter
•
All 56 bits can be read synchronously
•
Different 32-bit portions of the 56-bit counter can be read synchronously
•
Flexible service request generation based on compare match with partial STM
content
•
Driven by maximum 90 MHz (=
f
SYS
, default after reset =
f
SYS
/2)
•
Counting starts automatically after a reset operation
•
STM registers are reset by an application reset if bit ARSTDIS.STMDIS is cleared. If
bit ARSTDIS.STMDIS is set, the STM registers are not reset.
1)
•
STM can be halted in debug/suspend mode (via STM_CLC register)
Special STM register semantics provide synchronous views of the entire 56-bit counter,
or 32-bit subsets at different levels of resolution.
The maximum clock period is 2
56
×
f
STM
. At
f
STM
= 90 MHz, for example, the STM counts
25.39 years before overflowing. Thus, it is capable of continuously timing the entire
expected product life time of a system without overflowing.
14.2
Operation
The STM is an upward counter, running either at the system clock frequency
f
SYS
or at a
fraction of it. The STM clock frequency is
f
STM
=
f
SYS
/RMC with RMC = 0-7 (default after
reset is
f
STM
=
f
SYS
/2, selected by RMC = 010
B
). RMC is a bit field in register STM_CLC.
In case of an application reset, the STM is reset if bit SCU_ARSTDIS.DIS0 is set. After
reset, the STM is enabled and immediately starts counting up. It is not possible to affect
the content of the timer during normal operation of the TC1784. The timer registers can
only be read but not written to.
The STM can be optionally disabled for power-saving purposes, or suspended for
debugging purposes via its clock control register. In suspend mode of the TC1784
(initiated by writing an appropriate value to STM_CLC register), the STM clock is
stopped but all registers are still readable.
Due to the 56-bit width of the STM, it is not possible to read its entire content with one
instruction. It needs to be read with two load instructions. Since the timer would continue
to count between the two load operations, there is a chance that the two values read are
1) “STM registers” means all registers except STM_CLC, STM_SRC0, and STM_SRC1.
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