SH3000 Datasheet, PDF (10/24 Pages) Semtech Corporation – Low-Power Programmable Multifunction Support IC for Microcontrollers

SH3000 MicroBuddy™
SYSTEM MANAGEMENT
Real Time Support
The SH3000 has two support modules that are
specifically designed for various real time support
functions. They are the Real Time Clock and the
Periodic Interrupt / Wakeup Timer. Both of these units
as well as other functions of the SH3000 depend on the
internal 32 kHz clock for accuracy.
The SH3000 allows a trade-off between the cost of a
system and its accuracy.
For some devices, a single SH3000 without any
support components provides sufficient accuracy.
These units can operate with processor clock accuracy
of ±0.5% and the accuracy of the real time system of
±3%.
At the other end of the spectrum, with one external
component (a 32.768 kHz watch crystal), the SH3000
can provide a processor clock accuracy of ±256ppm
(±0.0256%) and the accuracy of the real time system of
±4ppm (±0.0004%).
Low Frequency (LF) Oscillator System
This module provides the 32 kHz clock to all internal
circuits and to the dedicated output pin, CLK32.
If enabled, the CLK32 output continues normal
operations when VDD is absent and backup power is
available.
When the power is first applied to the SH3000, the
RC oscillator takes over. It supplies the 32 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 kHz
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 power-up from the factory-programmed OTP
nonvolatile memory. They can be re-programmed at any
time or they can be permanently protected from any
changes by setting the Lock flag or an OTP write-protect
flag. Factory calibration brings the frequency of the RC
oscillator within ±3% of the 32768 Hz for the internal
reference resistor, and ±2% for the external
1M 1% resistor, over the entire temperature and supply
voltage range.
The frequency of the RC oscillator can be tuned or
modulated by varying the external reference resistor,
which should be located as close as possible to RREF,
pin 9.
32768 Hz
Watch Crystal
12.5 pF Load
Capacitance
VREG
X-TAL
RC
XIN
5
XOUT
6
CLKSEL
7
RREF
9
External
Reference
Resistor
1M 1% or
variable
VSS
Lock / Unlock
8
Logic
4-bit Value
VSS
1
X-tal stable?
INTERNAL
RREF ON
RC
Oscillator
Internal RREF
6-bit Value 4-bit Value Lock Logic
Figure 5. Simplified LF Oscillator System
CLK32 ON
Internal
32 kHz
Clock
From / To
Serial I/O
CLK32
13
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