What relations should be kept between the different VDD rails of EFR32 Series 1 for proper operation?
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.
What's the minimal BOM filtering for the EFR32 Series 1 Wireless Gecko?
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.
Where can I find EFR32 reference designs?
The EFR32 kit radio boards serve as the base reference designs. These can be found in a few locations:
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.
How do I use the CTUNE token for EFR32?
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.
What is the optimum load impedance for EFR32 Series 1 wireless gecko at 2.4GHz?
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.