NXP Semiconductors freescale semiconductor MC13211, Справочное руководство

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System Considerations
MC1321x Reference Manual, Rev. 1.6
Freescale Semiconductor 3-9
Load capacitors C3 and C4 are typically in the 5 pF to 25 pF range and are determined by the requirements
of the specific crystal or resonator. The manufacturer will specify the load capacitance and the capacitors
selected for C3 and C4 are used to match the stray circuit capacitance (the pin and PCB capacitance) to the
required load capacitance. As a first-order approximation, use 10 pF as an estimate of the combined stray
capacitance for a pin plus its PCB capacitance. The load capacitance is the series equivalent of (C3 +
Cextalstray) and (C4 + Cextalstray), where Cextalstray = pin capac trace capacitance. If C3 and
C4 get too large the frequency can be affected or the oscillator may not start reliably.
3.5.3.3 MCU Internal Clock Sources
Two internal MCU sources to provide system clock are provided such that the external clock sources are
not used at MCU start-up and may not be required for normal operation (see
Chapter 14, “MCU Internal
Clock Generator (ICG)” ). The two internal sources derive from an Internal Reference Generator, and one
or the other is engaged when the ICG is in Self-Clocked Mode (SCM):
• Approximate 8 MHz source - This source is not exact and is used to start the MCU from a reset or
stop condition in which the clocks had been stopped. This clock could be used as the system source,
but is not the best choice for accuracy and is not typically used other than at start-up.
• Internal reference clock (which can be trimmed), typically 243 kHz - The low frequency internal
reference is specified to be 243 kHz +/- 25% at start-up. However, this source can be trimmed to
+/- 0.2% resolution with +/- 0.5% across voltage. This internal reference can also be used with the
MCU frequency locked loop (FLL) to generate higher system clock speeds without need for an
external reference. This source provides a low power, low cost source with reasonable accuracy.
In addition to internal system clock sources, there is a third oscillator that is used only for a real-time
interrupt (RTI). RTI can be used to return from a stop condition while the MCU is in a low power mode
and can use an external clock or the internal RTI oscillator. The internal oscillator has a very low frequency
accuracy (a ratio from approximately 1 to 1.8), however, it has the lowest power. As a result, the internal
RTI oscillator can be used to determine long delays in non-critical applications where the battery life is
paramount compared to period accuracy. The Real-Time Interrupt is described in
Section 12.7.6, “System
Real-Time Interrupt Status and Control Register (SRTISC)” .
3.5.4 System Clock Configurations
Because of the multiple clock configurations of the MCU, the availability of external clock source pins of
both the modem and the MCU, and the CLKO output from the modem, there are a number of variations
for MC1321x system clock configurations. Key considerations for any system clock configuration are:
• The modem 16 MHz source (typically the crystal oscillator) must always be present. The crystal
has special requirements and the reference frequency must meet the 802.15.4 Standard.
• Battery-operated application requirements for low power can impact the choices for MCU clock
source.
• The system clock configuration will impact system initialization procedures.
• Software requirements can impact MCU processor and bus speed. The user must be aware of the
performance requirements for the MCU. The CPU clock is always 2X the internal bus speed, and
the application software such as the Freescale BeeStack and/or the 802.15.4 MAC often require