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Reduced Power Modes
2-35
MSP50C614 Architecture
The deepest sleep achievable on the C614, for example, is a mode where all
of the previously listed subsytems are stopped. In this state, the device draws
less than 10 µA of current and obtains the greatest power savings. It may be
awakened from this state using an external interrupt (input port).
A number of control parameters determine which of the internal components
are left running after the IDLE instruction. In most cases, the states of these
controls may be mixed in any combination. There are three combinations,
however, which are primarily useful. The three modes (light, mid, and deep
sleep) are executed through the independent control of two bits: 1) the idle
state clock control, and 2) the reference oscillator enable. The other pertinent
controls simply enhance the performance of the modes dictated by these two.
Table 2–3 gives a listing of all of the controls which should be maintained by
the programmer before engaging the IDLE instruction. In some cases, it will
be impossible to wake from sleep unless certain controls are set appropriately
before going to sleep. (In those cases, only the hardware RESET low-to-high
will bring the device back into its normal operating state.)
The top row in Table 2–3 lists the first of the two primary controls, namely, the
idle state clock control. The idle state clock control determines the status of the
master clock (MC) during sleep. Setting the idle state control causes the CPU
clock, the PLL clock circuitry, and the MC to stop after the next IDLE
instruction. Clearing the idle state control causes only the CPU clock to stop
after IDLE. The PLL clock circuitry governs the MC and determines its rate.
Whenever the PLL circuitry is suspended, therefore, the MC stops. The idle
state clock control is accessed at bit 10 in the ClkSpdCtrl register. (Refer to
Section 2.9.3,
Clock Speed Control Register
, for more information.)
The reference oscillator enable is the other control which selects between the
three reduced power modes listed in Table 2–3. This control may be one of two
bits, depending on which oscillator reference is implemented in circuitry. Refer
to Section 2.9.3,
Clock Speed Control Register
. When using the
resistor-trimmed oscillator (RTO), the reference oscillator enable appears as
bit 8 in the ClkSpdCtrl register. When using the crystal-referenced oscillator
(CRO), the reference oscillator enable appears as bit 9 in the ClkSpdCtrl
register. If both bits 8 and 9 are clear, then no reference oscillator is enabled.
If either of bits 8 or 9 are set, then the reference oscillator enable is considered
set. This enables the PLL circuitry to regulate to the reference frequency, 32
kHz (assuming the idle state clock control is clear). Whichever state the
reference oscillator is in before idle, it remains in that state (running or stopped)
after idle. If the reference oscillator is left running during sleep, however, it
comes at a cost to power consumption. (This may be a necessary cost if, in
your application, elapsed time needs to be monitored during sleep.)