Question

What relations should be kept between the different VDD rails of EFR32 Series 1 for proper operation?

Answer

The following power supply restrictions need to be followed on the EFR32 Series 1 devices:

• VREGVDD must be the highest voltage in the system for EFR32.

• VREGVDD = AVDD

• DVDD <= AVDD

• IOVDD <= AVDD

• RFVDD <= AVDD

• PAVDD <= AVDD

• DVDD > DECOUPLE

These relations above are strongly recommended to be followed for proper operation of EFR32 wireless gecko, and mainly due to the internal ESD diode structures and operation conditions. 

Question

What's the minimal BOM filtering for the EFR32 Series 1 Wireless Gecko?

Answer

Silicon Labs reference radio board designs typically use an extensive number of components for RF and VDD filtering to achieve the best possible RF performance at even the highest output power levels. However, a number of these elements can be eliminated from the design while still maintaining an acceptable RF performance. 

Minimal BOM VDD rail filtering:

- VREGVDD: 100nF; 10uF

- AVDD: 10nF; 10uF

- DVDD: 100nF

- IOVDD: 1uF

- DECOUPLE: 1uF

- RFVDD: 220nF

- PAVDD: 220nF

Depending on the RF carrier frequency the following additional filtering capacitors are recommended to be added to the RFVDD and PAVDD nets:

- 10pF for the 2.4GHz applications

- 56-100pF at the high bands below 1GHz (868-915MHz, typically)

- 220-470pF at the lower sub-GHz bands (169-470MHz) 

In the case of having the PA-, RF- and DVDD rails run from the internal on-chip DC-DC converter, the following series inductors (or ferrites) need to be added:

- 22nH on PAVDD at 2.4GHz

- 100-120nH on PA-, and RFVDD at the higher sub-GHz bands

- 220-470nH on PA-, and RFVDD at the lower sub-GHz bands

Additional details can be found in "AN933.1: EFR32 Series 1 Minimal BOM" application note.

Note: This KBA has been marked as deprecated. More updated KBA on this topic can be found here:

How to find schematics and other documentation for radio boards?

Question

Where can I find EFR32 reference designs?

Answer

The EFR32 kit radio boards serve as the base reference designs.  These can be found in a few locations:

• Simplicity Studio v4 install locally, at C:\SiliconLabs\SimplicityStudio\v4\offline\hwtools\boards, listed by BRD number.  Included here are the board reference manual, BOM, schematic, assembly drawing, and gerbers.
• With Simplicity Studio v4 running and the radio board of interest connected and selected, all the documents mentioned above are listed under the Documentation link in the Launcher view.

• Document Library under "Schematics/Layout".  Included here are the schematic and BOM.

Design source (schematic and layout) may be available upon request.  Silicon Labs is working to provide these in the locations mentioned above in the near future.

Other EFR32 reference designs, like IoT Solutions product-specific reference designs, can be found here.

Note: This KBA has been marked as deprecated. More updated KBAs on this topic can be found here:

HFXO Capacitor Bank (CTune) calibration on EFR32

CTUNE Access

Saving CTUNE value as manufacturing token

Question

How do I use the CTUNE token for EFR32?

Answer

The EFR32 38.4MHz crystal does not require external loading caps, as there is a tunable capacitor bank in the EFR32 that can be used instead.  Different tune values can be written to registers to observe the corresponding frequency offset, and the CTUNE token is used to store the value for use by the application.  Here in this example, we refer to nodetest commands.

Once your EFR32 device is programmed with nodetest, if you enter “tokdump” you should see the ctune token amongst the tokens reported:

[C354] MFG_CTUNE FF FF

Note that there are also nodetest commands "setCtune" and "getCtune" where you can set and read the CTUNE register values. This is helpful if you want to try multiple specific ctune values and measure their affect prior to writing the token. From the nodetest "help" output:

setCtune u4 - set CTUNE registers (CMU->HFXOSTEADYSTATECTRL_CTUNE and CMU->HFXOSTARTUPCTRL_CTUNE)

getCtune - get CTUNE register value

A default / erased value for MFG_CTUNE corresponds to using the nodetest default ctune register value of 0x142. The EFR32 reference manual provides a formula for ctune:

Sets oscillator tuning capacitance.

Capacitance on HFXTAL_N and HFXTAL_P (pF) = Ctune = Cpar + CTUNE<8:0> x 40fF.

Max Ctune 25pF (CLmax ~12.5pF)

CL(DNLmax)=50fF ~ 0.6ppm (12.5ppm/pF)

With this calculation, the default of 0x142 (322 decimal) and no loading caps installed (as is the case with the BRD4151A modules in the WSTK) corresponds to a capacitance of 12.8pF.

You can issue the nodetest command "tokWrite c354" to write this token:

tokWrite u2 - Write all data of a token (u2=creator code)

Similar set/get of CTUNE register values can be achieved with RAIL test as well.  See the RAIL test user documentation for additional details.

Setting CTUNE token can also be achieved via Simplicity Commander.  See the Simplicity Commander User Guide UG162 for additional details on setting tokens.

Question

What is the optimum load impedance for EFR32 Series 1 wireless gecko at 2.4GHz?

Answer

The optimum termination impedance is determined by load-pull measurements at the 2.4GHz frequency band. The optimum load impedance yields the best TX power efficiency, while the RX performance is also close to its optimum.

The EFR32 has a single-ended 2.4GHz RF pin for TX and RX (2G4RF_IOP). The other pin, 2G4RF_ION, has to be tied to the GND and cannot be interchangeable with 2G4RF_IOP.

The 2G4RF_IOP pin does not need to be pulled up to the VDD, since the 2.4GHz PA is supplied from a separate PAVDD pin, moreover the RF pin is DC-grounded. 

The optimum termination impedance at the single-ended 2G4RF_IOP pin is 23+j11.5 ohms in the whole 2.4GHz ISM band for higher than +10dBm power output. For lower output power levels this optimum impedance is 20+j10.6 ohms.   

Details are in AN930.1 application note.

The on board voltage regulator lends itself for use in powering more than just certain power pins on the EFR32 itself.  Typically the DC-DC is recommended for powering the EFR32 RFVDD, DVDD and sometimes the PAVDD power pins (for desired transmit power levels 13 dBm and lower).  There may also be custom applications where it is appropriate to use the DC-DC regulated output for powering additional EFR32 power supply pins or even other device power pins.  In cases where the DC-DC is to be used for powering peripherals and / or the IOVDD supply, the designer should:

• Review the sleep management requirements to be sure that power pins powered by the DC-DC are not required to be powered during the energy management modes that may be possible for the design application.
• Estimate the static current draw from the datasheet, using the uA/MHz numbers for the core and the typical current consumption numbers for the radio.
• Implement a few worst case hardware test cases to:
  • Ensure the static max current (radio tx + max core freq + max external load) is below the DC-DC's max current.
  • Ensure the dynamic current draw for any external loads doesn't cause the EFR to brown out.  Using an oscilloscope, verify there aren't significant droops in the output supply when the external load is enabled.  For example, if an external load looked like a large capacitor, the transient current might cause the DC-DC output to droop. 

Note that all validation testing has been at 1.8V and therefore datasheet parameters may be different for DC-DC outputs other than 1.8 V.

Some EFR32 Series 1 and Series 2 variants DC-DC default to Bypass mode coming out of RESET and remain in Bypass mode until software enables the DC-DC.  Bypass mode effectively shorts the main supply to the DC-DC output pin.  Review the datasheet for the particular EFR32 part number to verify the DC-DC default operation coming out of RESET.

If one of the peripheral devices is also an EFR32, do not power that EFR32’s DVDD pin, due to DVDD will short to AVDD internally upon exiting reset and cause a potential over-current situation for the source DC-DC.

For additional information about the EFR32 DC-DC, please review application notes AN948 and AN948.2.

When done, Silicon Labs recommends you submit your finalized design for review in the customer portal. 

Question

How do I connect a TCXO to an EFR32 Series 1 SoC?

Answer

A TCXO clock source can be used to supply the EFR32 main clock.  Reconfiguration of the HFXO register settings in the Clock Management Unit will be required for operation with a TCXO.  Consult the EFR32 Reference Manual for additional details regarding the following settings:

• CMU_HFXOCTRL - HFXO Control Register
  • HFXO Mode
    • Set this bit to 1 in order to allow the single-ended TCXO instead of the oscillator.
    • This is set to 0 by default, which selects the crystal oscillator.
  • CMU_OSCENCMD - Oscillator Enable/Disable Command Register
    • HFXOEN
      • Set this bit to 1 in order for the HFXC Mode register setting to take effect.
      • This is set to 0 by default, which disables the HFXO.
    • HFXODIS
      • Set this bit to 1 to reset the oscillator setting to default if / when not using the TXCO or crystal oscillator.
      • This is set to 0 by default.

• CMU_HFXOSTARTUPCTRL - HFXO Startup Control / CMU_HFXOSTEADYSTATECTRL - HFXO Steady State Control
  • CTUNE
    • To compensate the extra load, CTUNE will probably need to be lowered or zeroed out compared to a crystal.

Be sure to physically connect the TCXO to the HFXTAL_N pin for the above register settings.  The HFXTAL_P pin should be no-connect.

Question

Are all EFR32 series 1 parts of the same package variant pin-compatible?

Answer

    No. The pin-outs are dependent on frequency band: Sub-G Band, 2.4G Band, and dual-band (Sub-GHz and 2.4GHz). Using EFR32FG1 QFN48 as an example, per the datasheet (http://www.silabs.com/Support%20Documents/TechnicalDocs/EFR32FG1-DataSheet.pdf) section 6 shows the pin definitions per package type and variant. Figure 6.1 shows the QFN48 sub-GHz pin-out, Figure 6.2 shows the QFN48 2.4GHz pin-out, and Figure 6.3 shows the QFN48 dual-band pin-out. All 3 pin-outs differ for pin function of pin numbers 13 through 21. And other pins' definitions are the same. The pin definition differences are listed as below: