SH3003 datasheet(17/30 Pages) SEMTECH

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POWER MANAGEMENT

SH3003

POWER MANAGEMENT

This module provides the 32.768kHz clock to all internal

circuits and to the dedicated output pin, CLK32. If en-

abled, the CLK32 output continues normal operations

when VDD is absent and backup power is available.

When the power is ﬁrst applied to the SH3003, the RC

oscillator takes over. It supplies the 32.768 kHz clock for

start-up and initialization. However, if the CLKSEL pin is

set high, then the crystal oscillator is enabled. Once the

crystal has started and stabilized, the internal 32.768 Hz

clock switches to the very accurate crystal frequency; see

Figure 5.

Just like the VBO value for the Reset circuit, the default

calibration values for the RC oscillator are loaded on pow-

er-up from the factory-programmed non-volatile memo-

ry. They can be re-programmed at any time or they can

be permanently protected from any changes by setting

the Lock ﬂag or a write-protect ﬂag. Factory calibration

brings the frequency of the RC oscillator within ± % of the

32.768kHz for the internal reference resistor, and ±2% for

the external 1MΩ 1% resistor, over the entire temperature

and supply voltage range.

The frequency of the RC oscillator can be tuned or mod-

ulated by varying the external reference resistor, which

should be located as close as possible to RREF, pin 9.

Application Information (continued)

VREG

Oscillator

X-TAL

Internal

32.768

kHz Clock

X-tal stable?

External

Reference

Resistor

1 M 1% or

variable

Internal RREF

32.768 kHz

Crystal

12.5 pF Load

Capacitance

Figure 5. Simplified LF Oscillator System

CLK32

XIN

XOUT

CLKSEL

RREF

VSS

CLK32 ON

4-bit Value

6-bit Value

4-bit Value

Lock / Unlock

Logic

Lock Logic

INTERNAL

RREF ON

From / To

Serial I/O

The crystal oscillator has the useful feature of adjustable

load capacitors. It permits tuning of the circuit for initial

tolerance of the crystal (often ±20ppm) as well as an ad-

justment for the required load capacitance (with possible

variations from the PCB layout). While the oscillator was

designed for a crystal with a nominal load capacitance of

12.5pF, the circuit accommodates any value from ~7pF to

22pF (depending on parasitics of the layout). All of these

corrections can be performed when the part is already

installed on the PCB, in the actual circuit.

The default value for load capacitance (12.5pF) loaded on

power-up from the factory-programmed nonvolatile mem-

ory can be re-programmed at any time (following a secure

process of unlocking the load capacitance value register

and immediately writing a new setting), or it can be com-

pletely protected from any changes by a permanent write-

protect ﬂag. This adjustment can set the frequency of the

crystal oscillator to within ±4ppm of the ideal value. As

a reference, a typical 32.768kHz crystal changes its fre-

quency 4ppm for a 10°C change in temperature.

Since the temperature characteristics of crystals are well

known and stable, the host processor is free to implement

an algorithm for temperature compensation of the crystal

oscillator using the adjustable load capacitors, with re-

sulting accuracy of ±4ppm over the entire temperature

range.

Low Frequency (LF) Oscillator System

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