
EM358x
42
Rev. 0.4
5.3.1
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 5-6 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 5-6. High-Frequency RC Oscillator Specification
Parameter
Test conditions
Min
Typ
Max
Unit
Frequency at reset
6
12
20
MHz
Frequency Steps
0.3
MHz
Duty cycle
40
60
%
Supply dependence
Change in supply = 0.1 V
Test at supply changes: 1.8 V to 1.7 V
5
%
5.3.2
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 5-7 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
®
Cortex
TM
-M3
NVIC.
Table 5-7. High-Frequency Crystal Oscillator Specification
Parameter
Test conditions
Min
Typ
Max
Unit
Frequency
24
MHz
Accuracy
-40
+40
ppm
Duty cycle
40
60
%
Start-up time at max bias
1
ms
Start up time at optimal bias
2
ms
Current consumption
200
300
μA
Current consumption at max bias
1
mA
Содержание EMBER EM358 series
Страница 2: ...EM358x 2 Rev 0 4 ...
Страница 7: ...EM358x Rev 0 4 7 ...