Yes, but depending on the drive impedance of the I/Os pulling it up, Vcc may end up at whatever fraction of a volt FETs just start to turn on and the part draw noticeable current.
So for example something sitting on an I2C bus may jam the bus, rather than operate off of it.
I said ‘effectivly’ not that the were actually connected to ground.
The diodes do indeed connect the I/O pins of the device that has been powered down to the VCC of that device, but since the VCC of that device is no longer connected the internals of the device will now pull the pin down towards ground.
The effect, as @cslorabox suggests is that the pins such as I2C and SPI will be pulled down towards ground and often cease to operate correctly, for active devices still connected to such pins.
As far as I understand with I2C the two I2C lines (SDA & SCL) are connected to open collector transistor/open drain mosfet inside the device. I have assumed powering down the Vcc line on the device will send these two lines into high impedance and the device does not draw current from these lines.
Anyone able to comment if my assumption is correct.
Thinking about a bit more, I have only considered the line driver side, there is circuitry in the device to monitor the I2C lines and this could easily draw current when Vcc is removed.
I may test some of my sensors in the next few days.
That is absolutely incorrect for most parts.
The reason is that there are protection diodes from the inputs to the supply rails on most parts. If the input is allowed to forward bias that diode by being higher than the supply rail by a sufficient fraction of a volt, current will flow and it will start to try to lift the supply rail. The pullups on an I2C bus may not be enough to power the part, and let the I2C bus work again even for talking to other parts, but putting just a few kilo ohms across most nodes battery packs would lead to a very premature battery death.
Some parts rated for I/O voltage above the supply may not have this input structure. Or they may have it, but be rated for a higher voltages with a series resistor and to tolerate a certain amount of injection current, which from a battery consumption point of view is still a major problem.
Your options are basically:
Keep parts powered, but put them in a low power sleep mode
Take the entire I2C bus low, probably by driving its pullups from a GPIO or from the supply of the peripherals turned off.
Segment the I2C bus. The common pass-MOSFET level translator circuit also works when the low voltage (but not the high voltage) side is shut down along with its pullup. It will also work if the voltage are the same on both sides, just don’t apply it backwards to a voltage difference.
Use an unique I2C bus per target, possibly bit banged, so that you can power it down. Or use some other interface.
Find parts with an input structure that allows the I/O voltage to be higher than the supply not only without exceeding ratings, but without causing milliamp current draw
Thanks @cslorabox I need to review my power management circuits as I may be wasting energy.
I just realised I forgot to mention I do power down the I2C pull-up resistors as well.
While my existing sensors have a sleep mode I still switched the Vcc line as a precaution as I can’t guarantee future sensors will have the same capability. As a result I probably have my existing sensors in full sleep mode but could have problems in the future. Thanks again
In addition to @cslorabox’s comments, whenever I have built a circuit that requires some I2C parts to be left accessible (RTC or FRAM for instance) when other I2C parts are powered down (sensors, displays) I have used an I2C bus isolator, such as the TCA4311.
First make sure you have T-Beam V10, with earlier versions it’s not possible.
Power channels of AXP192 are used on T-Beam V10 as follows:
DCDC1 0.7-3.5V @ 1200mA max → OLED
DCDC2 → unused
DCDC3 0.7-3.5V @ 700mA max → ESP32 (keep this on!)
LDO1 30mA → GPS backup battery
LDO2 200mA → LORA
LDO3 200mA → GPS
You can cut off all above channels by software. Use the AXP202x library.
If you cut off DCDC3 you cut off the ESP32. This should be kept on during sleep.
If you cut off LDO1 GPS chip will loose it’s current almanach data, thus will need a cold start after restart. So this should be kept on, when GPS is used. If you don’t need GPS, you man cut LDO3 and take out the small backup battery.
If the LoRa device is the only SPI device you want to power down, you can get the sleep current for that down to 0.2uA (typical) by appropriate register writes.
Hi, as said here, https://github.com/lyusupov/SoftRF/wiki/Prime-Edition-MkII
I have T22_V1.0 20190612 board which is T-BEAM V10.
Then i try to connect VCC pin of both devices (Gyro and Pulsometer) to DCDC1, then perhaps could power down that pin
Yes, that should work. DCDC1 can drive up to 1200mA.
Don’t forget to program approriate voltage for DCDC1, depending on the devices you want to attach. Look at my power.cpp as example.
DCDC1 is wired to header 3V3 pins.
Thanks, 2 questions?
DCD1 can ouput 5Volts?
What is the AXP Primary Address of my board? 0x34 ??
ketch always say that initialization failed
I have this simple sketch, i think that at any moment i switch off (I hope no damage it) lora and i can’t switch on again (it outputs error):
17:21:06.903 -> FAILURE
17:21:06.903 -> /home/carlos/Arduino/libraries/arduino-lmic-master/src/lmic/radio.c:818
// Basic config
#include “axp20x.h”
#define AXP192_PRIMARY_ADDRESS (0x34)
AXP20X_Class pmu;
void AXP192_power(bool on) {
if (on) {
pmu.setPowerOutPut(AXP192_LDO2, AXP202_ON); // Lora on T-Beam V1.0
pmu.setPowerOutPut(AXP192_LDO3, AXP202_ON); // Gps on T-Beam V1.0
pmu.setPowerOutPut(AXP192_DCDC1, AXP202_ON); // OLED on T-Beam v1.0
// pmu.setChgLEDMode(AXP20X_LED_LOW_LEVEL);
pmu.setChgLEDMode(AXP20X_LED_BLINK_1HZ);
} else {
pmu.setChgLEDMode(AXP20X_LED_OFF);
pmu.setPowerOutPut(AXP192_DCDC1, AXP202_OFF);
pmu.setPowerOutPut(AXP192_LDO3, AXP202_OFF);
pmu.setPowerOutPut(AXP192_LDO2, AXP202_OFF);
}
}
void AXP192_init() {
if (pmu.begin(Wire, AXP192_PRIMARY_ADDRESS)) {
Serial.println("AXP192 PMU initialization failed");
}
else {
Serial.println(“Initialization…”);
// configure AXP192
pmu.setDCDC1Voltage(3300); // for external OLED display
pmu.setTimeOutShutdown(false); // no automatic shutdown
pmu.setTSmode(AXP_TS_PIN_MODE_DISABLE); // TS pin mode off to save power
// switch ADCs on
pmu.adc1Enable(AXP202_BATT_VOL_ADC1, true);
pmu.adc1Enable(AXP202_BATT_CUR_ADC1, true);
pmu.adc1Enable(AXP202_VBUS_VOL_ADC1, true);
pmu.adc1Enable(AXP202_VBUS_CUR_ADC1, true);
// switch power rails on
AXP192_power(true);
// I2C access of AXP202X library currently is not mutexable
// so we better should disable AXP interrupts... ?
#ifdef PMU_INT
pinMode(PMU_INT, INPUT_PULLUP);
attachInterrupt(digitalPinToInterrupt(PMU_INT), PMUIRQ, FALLING);
pmu.enableIRQ(AXP202_VBUS_REMOVED_IRQ | AXP202_VBUS_CONNECT_IRQ |
AXP202_BATT_REMOVED_IRQ | AXP202_BATT_CONNECT_IRQ |
AXP202_CHARGING_FINISHED_IRQ,
1);
pmu.clearIRQ();
#endif // PMU_INT
Serial.println("AXP192 PMU initialized");
}
}
void setup() {
Serial.begin(115200);
// put your setup code here, to run once:
AXP192_init();
AXP192_power(true);
}
void loop() {
}
and if you completely power down, some I2C devices need to be re-initialised (takes time too)
first thing to do after waking up … like coffee in the morning 
DCDC1 can be set 0.7 … 3.5V.
If you need 5V on T-Beam you must use VBUS, and cannot switch off.
0x34 is I2C address of AXP192.
Be careful to set appropriate voltage by
pmu.setDCDC1Voltage(3300)
and AFTER enable the channel by
pmu.setPowerOutPut()
If you can’t initialize the AXP192 chip you perhaps ran into i2c bus problem. Increase log level and watch out for i2c bus errors.
That’s why i use the OLED powersave mode in the paxcounter software, not shutdown. Otherwise i2c bus will crash 
When you “power down” your pull-up resistors, are you isolating each resistor from the other such that they are not loading the I2C via the two resistors effectively being connected in series across the I2C bus?
Good point.
No my resistors are still tied together and still connected to Vcc of the sensor. I power down all the sensors and pull-up resistors by open circuiting power supply line with a high side mosfet. Leaving Vdd and SDA and SCL still connected to the processor.
There’s nothing wrong with that, if you take both SDA and SCL low at the MCU. There would only be a problem if you kept them high, or too only one of them low. A resistor without a voltage difference across it doesn’t turn electricity into heat.
Thanks, I’m still trying to manage AXP, but i still can’t initialize it, while, i have a doubt, if i disconnect ground pin of 5V device (such a accelerometer) instead VBUS pin, is more dangerous for board? Disconnecting VBUS, device still maintains power led on, but low intentisty, and device not work, perhaps is getting power from SCL and SDA pin and i think it is dangerous. But if i disconnect GND pin, device switchs off completely.
Thanks, i’ll buy a step-up voltage converter, to raise voltage to 5v.
I’m still trying to work with AXP but still not initialize, i’m using a axp20.h library of m5stack, i can’t found it in your github PaxCounter, seems no longer uses that library, but it used in past. How can i get it? Thanks again!!!
If you are using the Arduino IDE, you need to install the AXP202x library by hand.
If you are using Platformio IDE (like paxcounter), the IDE will grab and install it for you.
Does it sure make sense to have a voltage conversion 5V to 5V like this?
5V USB -> VBUS -> DCDC (AXP192) -> 3V3 -> DCDC(MT3608) -> 5V