Auto Clock Detect Mode - Semtech SH3000 User Manual

5. The host may perform a custom or proprietary algorithm for frequency control; the SH3000
provides all of the information, reference, and timing signals needed.
During each of the settling methods described above, the processor clock can be switched to 32 kHz, and
would be completely free of any frequency fluctuations or instabilities. Since the SH3000 automatically
shuts down the HF oscillator when 32 kHz is selected as the output frequency, the HF oscillator should be
forced to an active state by setting the Force DCO On bit (bit 5) in the Config register (R0x0E).
Systems requiring a very stable clock for short periods of time (controlling an integrating D/A converter, for
example), may stop the FLL action in the SH3000 by resetting the FLL enable bit (bit 0) in the FLLcontrol
register (R0x0F).

3.1 Auto Clock Detect Mode

The SH3000 HF oscillator block has two main modes with which to operate.
1. Mode 1 provides a system clock source. In this mode the CLK
frequency configured by the register settings. It can be enabled/disabled by setting/resetting the
CLK pin enable bit (bit 3) of the Config register (R0x0E).*
OUT
2. Mode 2 provides a CPU oscillator source. In this mode, the CLK
frequency configured by the register settings. It can be enabled/disabled by setting/resetting the
CLK pin enable bit of the Config register.* When the host microcontroller enters stop mode,
OUT
the SH3000 CLK
shuts down the CLK
configures the CLK
interrupt signal, the CLK
clock within 2 ms. This happens much faster than with a standard crystal or a ceramic resonator,
faster even than with an LC tank circuit. This is a key feature of low-power operation of the
SH3000.
The operating mode is determined by CLK
pin, it assumes Mode 2 is in effect and configures itself to auto-clock detect. This also sets the
corresponding bit (bit 4) in the Status register (R0x1A).
* As programmatically disabling the CLK
must be set and enabled before this is done. The SH3000 does not disable the clock unless the periodic
timer is enabled.
SH3000UM version 0.95
Copyright ©2002 Semtech Corporation
pin detects the absence of transitions from the host oscillator output pin, and
IN
pin within four clock cycles to save power. At this time the SH3000
OUT
pin in such a way that as soon as the host exits stop mode via a reset or
OUT
pin receives a transition. When this occurs, the SH3000 restarts the
IN
. If the SH3000 detects at least four transitions on the CLK
IN
pin causes the clock to be irrecoverable, the periodic timer
OUT
2002-08
SH3000 User Manual
Preliminary
pin supplies a clock at the
OUT
pin supplies a clock
OUT
IN
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