EM357-RTR datasheet(42/240 Pages) ETC2

42 / 240 page

EM351 / EM357

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.

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 V to 1.7 V

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 6-7. High-Frequency Crystal Oscillator Specification

Parameter

Test conditions

Min

Typ

Max

Unit

Frequency

MHz

Accuracy

-40

+40

ppm

Duty cycle

Start-up time at max bias

Start up time at optimal bias

Current consumption

200

300

μA

6-7

120-035X-000D

Preliminary

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