Question

EFR32 Series 1 – What is the maximum value resistor to use for a GPIO pull-up / down?

Answer

Maximum external pull-up / down resistor value for Industrial rated devices operating in temperatures up to -40/85° C.

EFR32 (all GPIOs except PB14/PB15): 10 M ohms

EFR32 (PB14/PB15): 9.1 M ohms

Maximum external pull-up / down resistor value for High Temperature Industrial rated devices operating in temperatures up to +125° C.

EFR32 (all GPIOs except PB14/PB15): 4.12 M ohms

EFR32 (PB14/PB15): 1.8 M ohms

Use of an external pull-up or pull-down resistor instead of the internal pull-up or pull-down on an EFR32 GPIO provides an opportunity for reduced current draw in certain applications, particularly in battery powered designs. The reason for this is the maximum value resistance that works as a pull-up or pull-down for an EFR32 GPIO pin is much greater than the internal pull-up or pull-down resistance listed in the EFR32 datasheets (around 43k ohms), which equates to significant power savings in which one or more pull-up or pull-down resistors are frequently drawing current.  For example, a battery operated open / closed sensor switch may be required to have a pull-up or pull-down on the switch circuit.  The firmware may be monitoring for a high level on the switch / GPIO circuit.  In the case of a normally closed switch, when the switch is engaged, the resulting circuit to ground through the switch is detected by the firmware and acted upon. Depending upon how often or how long the switch is activated, the resulting power drain through the pull-up or pull-down could significantly shorten the overall lifespan of the battery.  In this type of situation, use a large value external resistor instead of the internal pull-up or pull-down configuration of the GPIO pin in order to reduce the current draw and extend battery life.

To arrive at 9.1M ohms / 10 M ohms / as the maximum safe resistor pull-up / down value to use in temperature range -40° C to 85° C, we first determine the GPIO maximum input leakage current.  Characterization testing of Engineering IC test lots have revealed an upper limit of 30 nA for industrial temperature rated devices and an upper limit of 110 nA for high temperature industrial rated devices.  The exception are GPIOs PB14/PB15 which are revealed to have an upper limit of 50 nA in the range of -40° C to 85° C, and 250 nA at 125° C. 

Refer to the datasheet table 4.37 for Input leakage current (IIOLEAK), Input low voltage (VIOIL) and Input high voltage (VIOIH)

 http://www.silabs.com/Support%20Documents/TechnicalDocs/EFR32MG1-DataSheet.pdf

Having determined the leakage current, we then calculate the minimum input high voltage for the GPIO by multiplying the minimum IOVDD Operating supply voltage, 1.62 V (table 4.2 of the datasheet) by 0.7 (Table 4.37), giving us 1.134 V (1.62 * 0.7 = 1.134 V).  Next, we determine the allowable voltage drop across the external pull-up by subtracting the minimum Input high voltage from the minimum IOVDD Operating supply voltage, (1.62 V – 1.134 V = 0.486 V). By using Ohm’s Law to divide the voltage drop of the external pull-up by the threshold leakage current value and multiplying the result by 0.95, we safely determine the appropriate pull-up resistor value with a 5% margin. For high temperature industrial range of -40° C to +125° C, (0.486 V / 0.000000110 A * 0.95 = 4.2 M ohms), we reduce the pull-up resistor value to 4.12 M ohms as the nearest commercially available resistor value. For the temperature range of -40° C to +85° C, (0.486 V / 0.00000003 A * 0.95 = 15.39 M ohms), we limit the pull-up resistor value to 10 M ohms. The reason is anything greater in resistance than 10 M ohms is simply not reliable as the circuit will be affected by the resistances of many contaminants such as moisture and dirt, etc. For EFR32 GPIO PB14/15, the equation is (0.486 V / 0.00000005 A * 0.95 = 9.2 M ohms), where we limit the pull-up resistor to 9.1 M ohms as the nearest commercially available resistor value.

Similarly, for the pull-down value we calculate the maximum input low voltage for the GPIO by multiplying 0.3 (table 4.37) by the minimum IOVDD Operating supply voltage, (0.3 V * 1.62 V = 0.486 V).  The result, 0.486 V, is the allowable voltage drop for the external pull-down resistor.  By using Ohm’s Law to divide the voltage drop of the external pull-down by the threshold leakage current value and multiplying the result by 0.95, we safely determine the appropriate pull-down resistor value with a 5% margin. For the high temperature industrial range of -40° C to +125° C, (0.486 V / 0.00000011 A * 0.95 = 4.2 M ohms), and for the temperature range of -40° C to +85° C, (0.486 V / 0.00000003 A * 0.95 = 15.39 M ohms), we again limit the pull-down resistor value to 10 M ohms for reliability.

The same formulas used above when applied to PB14/15 for the temperature range of -40° C to +125° C result in maximum pull-up/down resistor values of 1.8 M ohms.  (0.486 V / 0.00000025 A * 0.95 = 1.85M ohms).

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

How do I connect a TCXO to an EFR32?

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

EFR32 Series 1 – What is the maximum value resistor to use for a GPIO pull-up / down?

Answer

Maximum external pull-up / down resistor value for Industrial rated devices operating in temperatures up to -40/85° C.

EFR32 (all GPIOs except PB14/PB15): 10 M ohms

EFR32 (PB14/PB15): 9.1 M ohms

Maximum external pull-up / down resistor value for High Temperature Industrial rated devices operating in temperatures up to +125° C.

EFR32 (all GPIOs except PB14/PB15): 4.12 M ohms

EFR32 (PB14/PB15): 1.8 M ohms

Use of an external pull-up or pull-down resistor instead of the internal pull-up or pull-down on an EFR32 GPIO provides an opportunity for reduced current draw in certain applications, particularly in battery powered designs. The reason for this is the maximum value resistance that works as a pull-up or pull-down for an EFR32 GPIO pin is much greater than the internal pull-up or pull-down resistance listed in the EFR32 datasheets (around 43k ohms), which equates to significant power savings in which one or more pull-up or pull-down resistors are frequently drawing current.  For example, a battery operated open / closed sensor switch may be required to have a pull-up or pull-down on the switch circuit.  The firmware may be monitoring for a high level on the switch / GPIO circuit.  In the case of a normally closed switch, when the switch is engaged, the resulting circuit to ground through the switch is detected by the firmware and acted upon. Depending upon how often or how long the switch is activated, the resulting power drain through the pull-up or pull-down could significantly shorten the overall lifespan of the battery.  In this type of situation, use a large value external resistor instead of the internal pull-up or pull-down configuration of the GPIO pin in order to reduce the current draw and extend battery life.

To arrive at 9.1M ohms / 10 M ohms / as the maximum safe resistor pull-up / down value to use in temperature range -40° C to 85° C, we first determine the GPIO maximum input leakage current.  Characterization testing of Engineering IC test lots have revealed an upper limit of 30 nA for industrial temperature rated devices and an upper limit of 110 nA for high temperature industrial rated devices.  The exception are GPIOs PB14/PB15 which are revealed to have an upper limit of 50 nA in the range of -40° C to 85° C, and 250 nA at 125° C. 

Refer to the datasheet table 4.37 for Input leakage current (IIOLEAK), Input low voltage (VIOIL) and Input high voltage (VIOIH)

 http://www.silabs.com/Support%20Documents/TechnicalDocs/EFR32MG1-DataSheet.pdf

Having determined the leakage current, we then calculate the minimum input high voltage for the GPIO by multiplying the minimum IOVDD Operating supply voltage, 1.62 V (table 4.2 of the datasheet) by 0.7 (Table 4.37), giving us 1.134 V (1.62 * 0.7 = 1.134 V).  Next, we determine the allowable voltage drop across the external pull-up by subtracting the minimum Input high voltage from the minimum IOVDD Operating supply voltage, (1.62 V – 1.134 V = 0.486 V). By using Ohm’s Law to divide the voltage drop of the external pull-up by the threshold leakage current value and multiplying the result by 0.95, we safely determine the appropriate pull-up resistor value with a 5% margin. For high temperature industrial range of -40° C to +125° C, (0.486 V / 0.000000110 A * 0.95 = 4.2 M ohms), we reduce the pull-up resistor value to 4.12 M ohms as the nearest commercially available resistor value. For the temperature range of -40° C to +85° C, (0.486 V / 0.00000003 A * 0.95 = 15.39 M ohms), we limit the pull-up resistor value to 10 M ohms. The reason is anything greater in resistance than 10 M ohms is simply not reliable as the circuit will be affected by the resistances of many contaminants such as moisture and dirt, etc. For EFR32 GPIO PB14/15, the equation is (0.486 V / 0.00000005 A * 0.95 = 9.2 M ohms), where we limit the pull-up resistor to 9.1 M ohms as the nearest commercially available resistor value.

Similarly, for the pull-down value we calculate the maximum input low voltage for the GPIO by multiplying 0.3 (table 4.37) by the minimum IOVDD Operating supply voltage, (0.3 V * 1.62 V = 0.486 V).  The result, 0.486 V, is the allowable voltage drop for the external pull-down resistor.  By using Ohm’s Law to divide the voltage drop of the external pull-down by the threshold leakage current value and multiplying the result by 0.95, we safely determine the appropriate pull-down resistor value with a 5% margin. For the high temperature industrial range of -40° C to +125° C, (0.486 V / 0.00000011 A * 0.95 = 4.2 M ohms), and for the temperature range of -40° C to +85° C, (0.486 V / 0.00000003 A * 0.95 = 15.39 M ohms), we again limit the pull-down resistor value to 10 M ohms for reliability.

The same formulas used above when applied to PB14/15 for the temperature range of -40° C to +125° C result in maximum pull-up/down resistor values of 1.8 M ohms.  (0.486 V / 0.00000025 A * 0.95 = 1.85M ohms).