RELEASE
Revision B1
©Serious Integrated, Inc.
35
CLOCKS, OSCILLATORS, AND TIME KEEPING
The following terms are important to understand theory of operation described herein:
Clock:
A square-wave logic-level periodic signal (not a clock as in a timekeeping clock of
time/date/minutes/hours, etc.)
Oscillator:
A crystal or resonating oscillator circuit that creates a fixed-frequency sine wave used in a specific circuit
to create a clock signal
RTCC:
A real time clock/calendar which keeps track of the correct time/date as set by the end user or some other
means.
There are many producers and consumers of stable clock signals (or “clocks”) on the SIM535:
1)
The SH7269 is driven a 13.33 MHz clock signal, which is internally in the MCU multiplied by 20 to yield a 266MHz CPU clock
speed.
2)
For variants with audio capability, the SH7269 and audio DACs require an audio sampling clock. This clock is typically
24.576MHz (48 KHz * 512), but can be changed based on the desired sampling rate.
3)
Some variants have a 32.768 KHz crystal attached to an external RTCC chip. This RTCC chip, when correctly programmed via
its I2C port, can deliver a 32.768 KHz clock to the SH7269’s RTCC clock input.
4)
Both the SH7269’s
USB Device port
and the external
USB Host port
require a stable 12MHz clock to operate within USB
specifications.
HIGH SPEED (MHZ) CLOCKS
The high speed clock system is driven by a 12MHz oscillator feeding a
Silicon Labs Si5351C
clock synthesis chip which
turns the 12MHz input into 5 output clocks:
Schematic
Signal Name
Programmed
Frequency (MHz)
Clock
Consumer
CLKMCU_13MHz
13.333
CPU Core
CLKVIDEO_27MHz
27.000
Video Input (CPU) – not used
CLKAUDIO_24MHz
24.576
Audio Sampling (CPU, D/A)
CLKUSBF_12MHz
12.000
USB Device (CPU)
CLKUSBH_12MHz
12.000
USB Host (FT313)
These frequencies can be changed at runtime via the I2C control port on the Si5351C. See the
I2C Device Summary
and
the
Silicon Labs Si5351 Data Sheet
for details.
HIGH SPEED CLOCKS AND ELECTROMAGNETIC INTERFERENCE (EMI)
The
SH7269
MCU is a high performance, high speed device and hence may (as compared to lower speed logic) create
concerns on the part of the system design with regards to radiated electromagnetic interference (EMI). The EMI
characteristics of a SIM will change as it is installed in a specific customer configuration, and there are two main tools
available to the system designer to help manage EMI if issues do arise.
The Si5351C’s output frequencies can be software-adjusted to help minimize certain EMI frequencies that may cause
issues when mounted in a complete system, and the chip also provides for a spread-spectrum feature.
MCU REAL-TIME CLOCK/CALENDAR (RTCC)
The SH7269 MCU has an internal Real-Time Clock/Calendar (RTCC) peripheral, designed to be driven from an attached
32.768 KHz crystal or external 32.768 KHz clock source.
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