Every time I see this type of implementation of a resistor I tend to question it's use (the 4.7k in this case).

enter image description here

Consider a case where the RFM module is not connected to the 4.7k resistors. Clearly we have a voltage divider configuration that governs a 0V at the upper terminal of the 10k resistor when the output of the MCU is LOW. However, when the output of the MCU is HIGH, we would have approx. 3.4V at the output (which would generally be considered a logic HIGH as well). However, how does the connection of the RFM module affect this configuration?

Am I looking at this all wrong and the purpose of the two sets of resistors is not a V divider? As the pins of an IC generally HIGH-Z to not affect the V divider configuration shown above?

In general, what are these types of "bridging" resistors used for? I can see in certain applications they are used for current limiting (like the case of a resistor connecting the base of a transistor), but I have seen other instances where I am unsure.


1 Answer 1


These resistors indeed work as voltage dividers. This is done for level-shifting the logic signal from 0-5V to 0-3.3V .

The snippet in the O.P. doesn't show it, but I suspect that the μC is powered with +5V in this schematic. RFM128 is powered with +3.3V, and it isn't 5V-tolerant, probably.

Notice also that only the outputs from the μC (SS#, MOSI, SCK) go through voltage dividers. The input to the μC (MISO) does not go through the divider, because it has 0-3.3V levels, because it's generated by an IC powered with +3.3V.

  • \$\begingroup\$ Your last point was a question I had as well. However, I figured it was beyond the scope of this question because it was likely specific to this implementation. \$\endgroup\$
    – sherrellbc
    Feb 15, 2014 at 4:40
  • \$\begingroup\$ Can anything be said about the effect of the RMF's terminals on the divider circuit? Given a 4.7k and 10k configuration, 3.4V (which is above 3.3V too, wouldn't we have a problem with this?) will be seen at the output. However, what happens when the RFM module is connected can can it be predicted? \$\endgroup\$
    – sherrellbc
    Feb 15, 2014 at 4:42
  • \$\begingroup\$ 10kΩ || high-Z ≈ 10kΩ Connecting the voltage divider to a high-Z input should have a negligible effect on the voltage divider voltage. \$\endgroup\$ Feb 15, 2014 at 4:47
  • \$\begingroup\$ I figured as much, but the comment I made regarding the high-Z nature of the module's terminals was a guess. In general, are terminals that require interfacing typically high-Z in nature? Also, wouldn't signaling a 3.3V device with 3.4V damage it? \$\endgroup\$
    – sherrellbc
    Feb 15, 2014 at 4:48
  • 1
    \$\begingroup\$ @sherrellbc Your questions have reached a level of detail that will require you to stop watching videos and start analyzing datasheets. Pick a 3.3V part and study the Absolute Maximum specifications. Pick any part and study the d.c. characteristics of the pins. \$\endgroup\$
    – Joe Hass
    Feb 15, 2014 at 11:28

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