NXP Semiconductors freescale semiconductor MC13211, Reference Manual

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System Considerations
MC1321x Reference Manual, Rev. 1.6
3-24 Freescale Semiconductor
3.11.2 Modem Event Timer Block with Four Timer Compare Registers
The MC1321x contains an internal Event Timer block whose clock is derived from a programmable
prescaler which is driven by the 16 MHz crystal source. The Event Timer consists of the prescaler and a
24-bit counter which increment whenever the modem crystal clock is operating. The Event Timer provides
the following functions:
• The 24-bit counter generates “current” system time - The counter runs continuously when the
modem is not in a low power mode and provides a local “current” time. The present value can be
read from Current_Time_A Register 26, Bits 7-0, and Current_Time_B Register 27, Bits 15-0. The
counter can be programmed to be clocked at a rate varying from 15+ kHz to 2 MHz. The counter
value is used to generate a time stamp for received frames and is also used for comparison to four
different comparators that allow up to four different time delays
• Interrupt generation at pre-determined system times - There are four comparator functions that can
generate an interrupt to the MCU when the value of the comparator matches the system counter.
These function as timers to generate time delays or trigger events and can augment the MCU timers
for generating time delays required within the 802.15.4 application software
• Exit from Doze Mode at pre-determined system time - Timer comparator Tmr_Cmp2 can be used
to wake-up the modem from low power Doze Mode
• Latches “timestamp” value during packet reception - With every received frame, there is a
timestamp generated and stored in a modem SPI register. The timestamp is taken from the counter
when the FLI is received
• Initiates timer-triggered sequences - The modem has three basic active modes of RX, TX, and
CCA. Each of these sequences can be timer-triggered to become active after a set delay based on
timer comparator Tmr_Cmp2. This feature can be useful for implementing 802.15.4 application
sequences
Operation, loading, and reading of the modem timer registers is done through the SPI interface. Chapter 6,
“Modem Timer Information” , describes the modem timer operations in detail.
3.11.3 MCU Real-Time Interrupt (RTI)
The MCU has a real-time interrupt capability that can be used to generate periodic interrupts based on a
multiple of the source clock period. The RTI is programmable to have a period of 256, 1024, 2048, 4096,
8192, 16384, or 32768 times the source clock period. The RTI clock source can be:
• RTI Internal Clock Source - This source is independent of the ICG and allows lowest power
operation. The period of the internal clock is nominally 1 ms, however, it has a very loose tolerance
with a spec of 700 -1300
μ
S. The maximum RTI period then is 1.024 second nominal
• External Clock Source - This source uses an external crystal or clock source. For best accuracy and
longest RTI period, a 32.768 kHz clock crystal is commonly used which allows a maximum RTI
period of exactly 1 second. Use of the external clock requires slightly more power and is available
in run, wait, and Stop3 modes
The RTI is covered in detail in Chapter 12, “MCU Resets, Interrupts, and System Configuration” .