EM357-RTR datasheet(42/241 Pages) SILABS

42 / 241 page

Rev 1.3

6.3.1. High-Frequency Internal RC Oscillator (OSCHF)

The high-frequency RC oscillator (OSCHF) is used as the default system clock source when power is applied to

the core domain. The nominal frequency coming out of reset is 12 MHz and Ember software calibrates this clock to

12 MHz. Table 6.6 contains the specification for the high frequency RC oscillator.

Most peripherals, excluding the radio peripheral, are fully functional using the OSCHF clock source. Application

software must be aware that peripherals are clocked at different speeds depending on whether OSCHF or

OSC24M is being used. Since the frequency step of OSCHF is 0.3 MHz and the high-frequency crystal oscillator is

used for calibration, the calibrated accuracy of OSCHF is ±150 kHz ±40 ppm. The UART and ADC peripherals may

not be usable due to the lower accuracy of the OSCHF frequency.

6.3.2. High-Frequency Crystal Oscillator (OSC24M)

The high-frequency crystal oscillator (OSC24M) requires an external 24 MHz crystal with an accuracy of ±40 ppm.

Based upon the application’s bill of materials and current consumption requirements, the external crystal may

cover a range of ESR requirements. Table 6.7 contains the specification for the high frequency crystal oscillator.

The crystal oscillator has a software-programmable bias circuit to minimize current consumption. Ember software

configures the bias circuit for minimum current consumption.

All peripherals including the radio peripheral are fully functional using the OSC24M clock source. Application

software must be aware that peripherals are clocked at different speeds depending on whether OSCHF or

OSC24M is being used.

If the 24 MHz crystal fails, a hardware failover mechanism forces the system to switch back to the high-frequency

RC oscillator as the main clock source, and a non-maskable interrupt (NMI) is signaled to the ARM® CortexTM-M3

NVIC.

Table 6.6. High-Frequency RC Oscillator Specification

Parameter

Test Conditions

Min

Typ

Max

Unit

Frequency at reset

MHz

Frequency Steps

—

0.3

—

MHz

Duty cycle

—

Supply dependence

Change in supply = 0.1 V

Test at supply changes: 1.8 to 1.7 V

—

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