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EM357-RTR 数据表(PDF) 44 Page - Silicon Laboratories |
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EM357-RTR 数据表(HTML) 44 Page - Silicon Laboratories |
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44 / 241 page  44 Rev 1.3 6.3.4. Low-Frequency Crystal Oscillator (OSC32K) A low-frequency 32.768 kHz crystal oscillator (OSC32K) is provided as an optional timing reference for on-chip timers. This oscillator is designed for use with an external watch crystal. When using the 32.768 kHz crystal, you must connect it to GPIO PC6 and PC7 and must configure these two GPIOs for analog input. Alternatively, when PC7 is configured as a digital input, PC7 can accept an external digital clock input instead of a 32.786 kHz crystal. The digital clock input signal must be a 1 V peak-to-peak sine wave with a dc bias of 0.5 V. Refer to "7. GPIO (General Purpose Input/Output)" on page 50 for GPIO configuration details. Using the low-frequency oscillator, crystal or digital clock, is enabled through Ember software. Table 6.9 contains the specification for the low frequency crystal oscillator. 6.3.5. Clock Switching The EM35x has two switching mechanisms for the main system clock, providing four clock modes. Table 6.10 shows these clock modes and how they affect the internal clocks. The register bit OSC24M_CTRL_OSC24M_SEL in the OSC24M_CTRL register switches between the high- frequency RC oscillator (OSCHF) and the high-frequency crystal oscillator (OSC24M) as the main system clock (SYSCLK). The peripheral clock (PCLK) is always half the frequency of SYSCLK. The register bit CPU_CLKSEL_FIELD in the CPU_CLKSEL register switches between PCLK and SYSCLK to produce the ARM® CortexTM-M3 CPU clock (FCLK). The default and preferred mode of operation is to run the CPU at the higher PCLK frequency, 24 MHz, to give higher processing performance for all applications and improved duty cycling for applications using sleep modes. In addition to these modes, further automatic control is invoked by hardware when flash programming is enabled. To ensure accuracy of the flash controller’s timers, the FCLK frequency is forced to 12 MHz during flash programming and erase operations. Table 6.9. Low-Frequency Crystal Oscillator Specification Parameter Test conditions Min Typ Max Unit Frequency — 32.768 — kHz Accuracy At 25 ºC –20 — +20 ppm Load capacitance OSC32A — 27 — pF Load capacitance OSC32B — 18 — pF Crystal ESR — — 100 kΩ Start-up time — — 2 s Current consumption At 25 °C, VDD_PADS=3.0 V — — 0.5 μA Table 6.10. System Clock Modes OSC24M_CTRL_OSC24M_ SEL CPU_CLKSEL_ FIELD SYSCLK PCLK FCLK Flash Program/ Erase Inactive Flash Program/ Erase Active 0 (OSCHF) 0 (Normal CPU) 12 MHz 6 MHz 6 MHz 12 MHz 0 (OSCHF) 1 (Fast CPU) 12 MHz 6 MHz 12 MHz 12 MHz 1 (OSC24M) 0 (Normal CPU) 24 MHz 12 MHz 12 MHz 12 MHz 1 (OSC24M) 1 (Fast CPU) 24 MHz 12 MHz 24 MHz 12 MHz |
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