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I'm using a PIC24F to interface with an SD card in SPI mode. When the card is idle, the device still draws ~200 uA of current which is unacceptable for this device's purpose. To correct this, I am using a bit-enabled Linear Regulator (NCP694H33HT1GOSCT-ND) to kill power to the SD card's power rail. However, the leakage current remained at ~200 uA even when the SD card's power was cut off. Then I realized that upon physically disconnecting the wires for the SPI clock and Data In to the SD card from the PIC, the current drops down to ~10 uA, which is very acceptable.

So, I am assuming that power is somehow leaking through the PIC24F and the signal lines to the SD card. I have tried reconfiguring those pins as digital outputs set to 0 and as digital inputs, but neither methods have had the same results as physically disconnecting those two lines.

Is there something I'm missing here? I would think that being set as a digital inputs would result in those pins becoming high impedance and thus blocking out the excessive leakage current, but it doesn't seem to be working. Any help would be appreciated.

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I believe your assumption is correct in that the card is leaking current from the signal lines.

Did you ensure that the PIC did not have its weak pull-ups active when you modified the port settings?

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  • \$\begingroup\$ I am using macros (which have previously worked in correctly setting up ports) to modify the port settings. For example, DISABLE_RA0_PULLUP(); sets the _CN2PUE register bit to 0. Then I set _TRISA0 = 1 and _PCFG0 = 1. Does that seem correct? Are the PIC's pull-ups notoriously weak? I was unaware of that. \$\endgroup\$ – James Feb 6 '11 at 22:38
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    \$\begingroup\$ You want the pull-ups to be weak if you're not disabling them. The problem is that you're trying to bring the SCK and MOSI lines higher than their (0V) VCC, so there's a current path from VCC, through the internal pullups, and out through the diodes internal to the SD card. \$\endgroup\$ – Kevin Vermeer Feb 7 '11 at 15:26
  • \$\begingroup\$ Well, I am definitely disabling the pull-ups, but it is not correcting the issue. There is still a significant amount of current leakage through the SCK and MOSI lines. \$\endgroup\$ – James Feb 17 '11 at 20:18
  • \$\begingroup\$ Is the leakage equal between SCK and MOSI, or more so one than the other? \$\endgroup\$ – Adam Lawrence Feb 18 '11 at 3:44
  • \$\begingroup\$ Well, I tested them simultaneously with 2 different types of ammeters and I'm afraid that they may have had slightly different impedances. I was getting ~80mA on one line and ~120mA on the other. However, I can't remember which is which. I assumed they are very close to equal. \$\endgroup\$ – James Feb 18 '11 at 15:13

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