Magnus Errata Part 3

This article is the fourth in a two part series about some serious errata published regarding the Microchip PIC32 famility of embedded microcontrollers. Previous articles have been Magnus Errata Part 1,  Magnus Errata Part 2, and Magnus Errata Update. There has been a significant passage of time and with the arrival of new errata data, a new chapter in the story is called for. At the same time, a goal for this article is to clarify the application of errata data beyond the PIC32 designation. This is to reflect the fact that the PIC32 family is implemented in multiple varieties, each with unique characteristics.

The PIC32 family can be broadly divided into three main branches:

  1. Smaller parts: PIC32MX1xx/2xx
  2. Mid sized parts: PC32MX3xx/4xx
  3. Larger parts: PIC32MC5xx/6xx/7xx

The first condition was an issue with spurious Data Bus Exceptions when data constants were read from flash memory with interrupts turned on. The respective, current errata notes for this issue are:

1xx/2xx 3xx/4xx 5xx/6xx/7xx
Not applicable to these parts.

It is noteworthy that corrected silicon exists for the 5xx/6xx/7xx family but not the 3xx/4xx and that this issue does not apply to the 1xx/2xx family. I had hoped the errata would say more, but it does not. At the 2012 Master’s Conference I learned that this issue has never been observed when the cache and prefetch are set up normally. This explains why I have never seen this issue and why the vast majority never see it. If you are really paranoid, then implement the work-around, but for the majority of cases, I don’t see the point (PS: That’s my opinion, NOT engineering advice!)

The second condition was an issue with a double data write with interrupts turned on. This was indicated as mostly harmless except when the write target changed it’s state based on a write, like a UART, SPI port etc. The errata were:

1xx/2xx 3xx/4xx 5xx/6xx/7xx

Again, only the 5xx/6xx/7xxx family of parts has a note regarding a corrected revision, so this fix must still be forthcoming in the other two families. Also note that turning off interrupts during the write is now feature in addition to using DMA. And a correction to the errata is that the I/O port Toggle registers where left out as potential trouble spot. Double writing the Toggle register would create a very short spike rather than toggling the desired data bits.

One final word is that this article can never be the final word. I urge everyone working with a microcontroller to go to that chip’s web page and download and carefully study the published errata on that chip. As always; it’s what you don’t know that can hurt you.

Don’t forget that your comments and suggestions are most welcomed.

Peter Camilleri (aka Squidly Jones)

4 thoughts on “Magnus Errata Part 3

  1. Let me start by giving an apology for my very slow response to this comment. I will try to do better. This is more than I can say for these PIC chips however. A quick check of the relevant errata reveals that nothing has changed on these serious issues since my last posting. My guess is that the company feels that the workarounds in place are adequate and is instead focusing on new parts. That’s not official, just my perspective on things.

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