SH3000 Datasheet, PDF (9/24 Pages) Semtech Corporation – Low-Power Programmable Multifunction Support IC for Microcontrollers

SH3000 MicroBuddy™
SYSTEM MANAGEMENT
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 SH3000 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
SH3000 may 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 SH3000 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 3 for operating and performance
figures in the 8-16 MHz range.
Table 3. EMI reduction with Spectrum Spreading
Setting
Spreading Peak EMI Peak EMI
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 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).
Special Operating Modes
The SH3000 can operate stand-alone, without
connections to the In and Out terminals of the host’s
oscillator. For example, a bank of SH3000 chips can
generate several different frequencies for simultaneous
use in the system, all controlled by a single micro (and
possibly sharing one 32.768 kHz crystal by chaining the
CLK32 pin to XIN pin on the next device). In this case
the CLKIN pin should be connected to VSS. The clock
output on the CLKOUT pin is continuous; the correct
operating mode is automatically recognized by the
SH3000.
Likewise, a microcontroller may not have a STOP
command at all. Still, with the help of the SH3000 this
controller can do a “simulated” STOP by issuing an
instruction to the SH3000 to stop the clock. This
command is accepted only if the Periodic Interrupt /
Wakeup Timer has started (otherwise, once the system
is put to sleep, it would never wake up again). This
mode of operations is only possible if the host processor
is capable of correct operations with clock frequency
down to zero, and keeps all of the internal RAM alive
while the clock is stopped.
Copyright ©2003-2005 Semtech Corporation
9
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