Member Since: September 19, 2011

Country: United States

  • When a pull-up is used with the simple inline resistor method shown above the 5V device sources current to the 3.3V supply. If we assume the internal clamping diode of the 3.3V device clamps CS to 4V (3.3+0.7V) then 70uA (0.7V/10k) of current flows to the 3V3 supply. If the load on the 3V3 supply is less than 70uA (in this case) the voltage regulator must sink current - its normal application is to source current.

    I inadvertantly recreated this scenario using a 74LVC244 level-shifter by wiring a pull-up to 3.3V instead of 5V. The TC1262 regulator couldn't sink the 17uA of current and the 3.3V supply gradually increased to 5V. I didn't observe this problem with either the LD1117-3.3 or MCP1702 regulators so they seem to be able sink 17uA without problems.

  • Just tested the sleep command. You must ensure that you send "sleep", followed by a carriage return. If you use a carriage return and a newline character then the unnecessary trailing newline character immediately wakes up the RN-XV.

  • I tested sleep mode on a breadboard before making my RN-XV+SD shield. Sleep mode worked fine using the GPIOs. I think I also successfully tested sleep/wakeup using the sleep command and CTS. I wasn't using ad-hoc mode.

    Did you save and reboot after making configuration changes? What firmware version do you have? Sleep using GPIO8 needs version >=2.23, but the manual I have states "NOTE: Currently, the alternative GPIO functions are not available in adhoc mode."

  • I'm now using a 74LVC244 to translate the MOSI, SCK and CS signals to 3.3V levels. The 75LVC244 is powered from 3.3V with the output enable pins permanently grounded. I chose this device because the data sheet states its inputs allow 5V logic levels when powered from 3.3V, and it is also available in a DIP package.
    For each 3.3V SPI slave I'm using one gate from a 74HCT125 quad buffer to protect MISO from 5V SPI slaves. The buffer's output enable pin is connected to the slave's (active-low) CS input. The 74HCT125 is powered from 5V.
    This set up properly protects both inputs and outputs of 3.3V devices. It still works if the 5V logic levels are replaced by 3.3V ones, allowing custom Arduino shields to be used on both 5V and 3.3V Arduinos.

  • We have a limited quantity of these.
    Is this still the case? I'm designing my own PCB and I don't want to use items I can't get hold of in 6 months time.

  • There's no protection on the MISO pin of the 3.3V device, unnecessary when only 3.3V slave devices are used. However, when mixing 3.3V SPI slave devices with 5V slave devices there will be 5V signals on MISO. What is the best way to protect the 3.3V devices in this case?

No public wish lists :(