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Reduced Power Modes
2-36
The power consumed during sleep when the RTO oscillator is left running is
greater
than the power consumed during sleep when the CRO oscillator is left
running.
If the idle state clock control is clear, then the PLL circuitry, active during sleep,
will attempt to regulate the MC to whatever frequency is programmed in the
PLL multiplier (see Section 2.9.3,
Clock Speed Control Register
). The MC con-
tinues to run at this frequency, even during sleep, provided that the reference
oscillator is enabled.
If the idle state clock control is set, then neither the MC, CPU clock, nor the
TIMER clocks run during sleep, unless the TIMER source is linked to the
reference oscillator (Section 2.8, Time Registers). These relationships are
shown explicitly, as a function of the reduced power mode, in Table 2–4.
Because the DAC circuitry is the single most source of power consumed on
the C614, it is important to disable the DAC entirely before engaging any IDLE
instruction. This is accomplished at the DAC control register, address 0x34.
Refer to Section 3.2.2,
DAC Control and Data Registers
.
The ARM bit is another important control to consider before engaging the
reduced power mode. It is recommended that the ARM bit be cleared
whenever the idle state clock control is clear, and set whenever the idle state
clock control is set Table 2–3. The set ARM bit causes an asynchronous
response to all programmable interrupts when in the sleep state. (The cleared
ARM bit yields the standard synchronous response at all times.) Affected
interrupts include those tied to TIMER1 and TIMER2, as well as those tied to
the inputs at Ports F, D
2
, D
3
, D
4
, and D
5
. The advantage to having the ARM
bit set is that the device may be awakened by one of these interrupts, even
when the PLL clock circuitry is stopped in sleep (by virtue of the idle state
control). The disadvantage of the asynchronous response, however, is that it
can render irregularities in the timing of response to these same inputs.
Note: Idle State Clock Control Bit
If the idle state clock control bit is set and the ARM bit is clear, the only event
that can wake the C614 after an IDLE instruction is a hardware RESET low-
to-high. When at sleep, the device will not respond to the input ports, nor to
the internal timers.