SH3001 Datasheet, PDF (6/20 Pages) Semtech Corporation – Real-Time Clock and Clock Management Support IC for Microcontrollers

SH3001 MicroBuddy™
SYSTEM MANAGEMENT
When the HF oscillator is operating alone, it can set
the frequency of the clock on the CLKOUT pin to
± 0.025%, and maintain it to ± 0.5% over temperature.
This compares favorable with the typical ± 0.5% initial
clock accuracy and ± 0.6% overall temperature stability
of ceramic resonators. The SH3001 replaces the typical
resonator, using less space and providing better
performance and functionality.
The HF oscillator can also be locked to the internal
32.768 kHz signal. The absolute accuracy and stability
of the HF clock depends on the quality of the 32.768 kHz
internally generated clock; the low-frequency (LF)
Oscillator System is described later in this document.
When the Real-Time Clock module of the SH3001 is
used for high-accuracy timekeeping, an external
32.768 kHz crystal used as a reference for RTC provides
excellent accuracy and stability for the Clock
Management System.
The SH3001 employs a Frequency Locked Loop
(FLL) to synchronize the HF clock to the 32.768 kHz
reference. This architecture has several advantages
over the common PLL (Phase Locked Loop) systems,
including the ability to stop and re-start without
frequency transients or instability, and with instant
settling to a correct frequency. The conventional PLL
approach invariably includes a Low-Pass Filter that
requires a long settling time on re-start.
The primary purpose of the FLL is the maintenance
of the correct frequency while the ambient temperature
is changing. As the temperature drift of the HF oscillator
is quite small, any corrective action from the FLL system
is also small and gradual, commensurate with the
temperature variation.
The FLL system in the SH3001 is unconditionally
stable.
To set a new frequency for the FLL, the host
processor writes the 13-bit Frequency Set value. The
resulting output frequency is calculated using simple
formulas [1] and [2] (reference frequency is 32.768 kHz):
FOSC = 2048 Hz * (Frequency Set value + 1) [1]
FOUT = FOSC / (Post-divider setting) [2]
For example, a post-divider setting of ÷8 and the
Frequency Set value of 4000 (0x0FA0) produce an
output frequency of 1.024 MHz.
Programmable Spectrum Spreading
Most commercial electronic systems must pass
regulatory tests in order to determine the degree of their
Electromagnetic Interference (EMI) affecting other
electronic devices. In some cases compliance with the
EMI standards is costly and complicated.
The SH3001 offers a technique for reducing the
EMI. It can be a part of the initial design strategy, or it
can be applied in the prototype stage to fix problems
identified during compliance testing. This feature of the
SH3001 can greatly reduce the requirements for
radiofrequency shielding, and permits the use of simple
plastic casings in place of expensive RFI-coated or
metal casings.
The SH3001 employs Programmable Spectrum
Spreading in order to reduce the RF emissions from the
processor’s clock. There are five (5) possible settings;
please see Table 1 for operating and performance
figures in the 8–16 MHz range.
Table 1. EMI reduction with Spectrum Spreading
Spreading Peak EMI Peak EMI
Setting
Bandwidth Reduction Reduction
(guaranteed) (measured)
En CFG1 CFG0 ^kHz
db
db
0X
X
Off
0
0
10
0
32
-3
-3
10
1
64
-6
-7
11
0
128
-9
-10
11
1
256
-12
-15
Spectrum Spreading is created by varying the
frequency of the HF oscillator with a pseudo-random
sequence (with a zero-average DC component). The
Maximum-Length Sequence (MLS) 8-bit random number
generator, clocked by 32.768 kHz, is used. Only 4, 5, 6,
or 7 bits of the generated 8-bit random number are used,
according to the configuration setting.
Maximum fluctuations of the frequency depend on
the selected frequency range and the position within the
range. Selecting the HF oscillator frequency to be near
the high end of the range limits the peak variations to
± 0.1%, ± 0.2%, ± 0.4%, or ± 0.8% (corresponding to the
configuration setting).
Copyright ©2002-2005 Semtech Corporation
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