A Shock {Hardware} Bug in Raspberry Pi’s RP2350 Results in Sudden Pull-Down Habits

Raspberry Pi has confirmed a bug within the new RP2350 microcontroller household, which causes pins to freeze outputting 2.15V when configured as inputs utilizing the inner pull-down resistors — tied, it appears, to adjustments made by a vendor to an off-the-shelf fault tolerant pad IP block.

“[I] discovered a silicon bug,” Harmful Prototypes’ Ian Lesnet explains of the problem, which has been confirmed as an erratum in Raspberry Pi’s official documentation for the newly-launched dual-architecture RP2350 microcontroller household. “When a GPIO [General-Purpose Input/Output] pin is an enter with the pull-down resistor enabled, it acts like a bus maintain. We use the pull-down on the button, which connects to three.3V when pressed. In the course of the self-test urgent the button works, however then it by no means goes low once more, it sits at 2.15V…”

The Raspberry Pi RP2350 household is an bold second-generation in-house microcontroller design, and one which blends — for the primary time within the firm’s historical past — off-the-shelf Arm cores with free and open-source Hazard3 RISC-V cores. The latter have been developed by Luke Wren, principal {hardware} engineer at Raspberry Pi, and usually are not the supply of the issue — which is, as an alternative, being pinned on an unnamed IP vendor.

“We did not modify the [faulty] pad, we requested the seller to switch their very own pad,” Wren explains of the supply of the issue, which was traced to the analog circuitry of a specific fault tolerant (FT) pad. “There was one explicit construction on the RP2040 FT pad that restricted its tolerance, however on inspection the modified structure we obtained again was a very totally different circuit. It was a little bit of a blind spot for us in simulation, as a result of the provided simulation mannequin clearly doesn’t have this concern.”

That concern, as detailed in Raspberry Pi’s RP2350 datasheet as erratum RP2350-E9, is described as “latching conduct on Financial institution 0 pull-down resistors,” and impacts all GPIO pins: when configured as an enter that makes use of the inner pull-down resistor and with the output buffer disabled, a GPIO pin will latch at between 2.1-2.2V quite than be pulled again to floor — however solely after its first use.

As the problem is within the chip’s {hardware}, there is no straightforward repair — with Raspberry Pi stating it would tackle the problem via “documentation” quite than any adjustments to the software program improvement package (SDK). This consists of the advice to solely allow the enter buffer instantly earlier than studying, then disable it once more afterwards — although, for individuals who aren’t tied in to a specific circuit structure, a better repair is to configure inputs to make use of the inner pull-up resistors as an alternative, which aren’t affected.

In Lesnet’s case, the problem was labored round in two methods for the upcoming RP2350-powered Bus Pirate 5XL and Bus Pirate 6. “Buttons have been modified to make use of pull-ups, however we additionally depend on the pull-downs for open collector bus varieties,” he explains. “The pull-down holds the IO pin low so the PIO [Programmable Input/Output] would not want to govern the GPIO path. Boards have 100k pull-downs put in (as an alternative of 1M Ohm) till there’s a answer.”

Lesnet’s full thread is obtainable on Mastodon, as is Wren’s remark; extra particulars can be found within the RP2350 datasheet, web page 1340.