Why does old PIC draw too much current?

Question

I pulled an old PIC from the PIC24 family out of storage, probably last used about a year ago. I reprogrammed the device with a bare minimum program. It seems to work fine on all counts except that it pulls 55 mA from the Vdd pin and the current exits through Vss. All other pins seem to draw current in the uA range.

• No peripherals are enabled
• Input pins are not floating
• Output pins are not drawing current

This current draw is way out of specification for this device which should draw about 2mA. What would cause such high current to be drawn on a device that is otherwise operating normally?

Answer 1

You say that 55 mA is "way out of specification", but you provide no evidence of that. It seems to me you haven't bothered to look in the datasheet. You also haven't said which PIC 24.

I grabbed a random PIC 24H datasheet and looked up Vdd current. The PIC 24HJ12GP201, for example, can draw up to 90 mA at 3.3 V and 25°C when running at 40 MIPS. 55 mA is therefore not at all obviously "way out of specification".

READ THE DATASHEET.

Answer 2

You actually don't know whether the chip operates normally, because the space of functionality of what the chip can do is quite large; you have not executed an extensive test suite of all its functions. What can be said is that it works normally for your "hello, world" test case that you're running on it.

It does sound like some area of the silicon is leaking a lot of current through. It could be some on-chip peripheral, for instance. The IC has many circuits on it that are connected between VDD and VSS; if some of them leak, that might not impair the chip's ability to be programmed and to execute a "hello, world" program.

Answer 3

This is less than 1/4 watt at 3.3V. If the chip is operating at a high speed, performing even seemingly simple calculations, and no power saving measures are enabled, then a 24 series PIC processor can certainly dissipate a quarter watt running around in circles.

If you provide the chip part number, the program, and schematic, we might be able to identify specific reasons it's consuming more than the 2mA you expect.

If you want to investigate further

• Consider replacing it with a new chip and a similar program to compare the two.
• It may have a MAC and a divider. If your code is exercising these features heavily you'll consume a surprising amount of current
• Some peripherals consume current when not specifically configured - go through and disable the peripherals and peripheral clock
• Some older chips had rather high current consumption for certain blocks, like the brownout detector. Disable these in the fuse settings.
• Perform a complete chip erase and reset.
• Disconnect any programmers/debuggers to disable JTAG and related blocks.
• Ensure inputs aren't floating - even though the risk of damage is all but gone these days, floating inputs can draw significant currents operating in the linear region.