Watchdog Power Requirement for EN50271 standard (STM8L152)

Chris Reynard · ‎2018-06-13

Posted on June 13, 2018 at 12:15

Hello,

I am looking at using a STM8L152 MCU in a product that detects and measures a combustible gas.

There are standards that govern how these products must operate, one of which is EN 50271:2010.

STM8L152 has a built-in watchdog feature but for this to satisfy the 'monitoring equipment' requirements of the standard it must operate separately from the main data processing function. This includes the voltage supply. It is okay for the monitoring equipment and data processing function to be connected to the same power supply but they should be connected 'separately'. This is not easy to demonstrate when the watchdog and main data processing function are packaged together.

I have looked through various published datasheets/technical docs/ref manual but have not come across detail which would provide assurance that this requirement is being met.

The main data processing function derives power from a power regulator (which in turn is fed from Vdd1,Vss1). If there is a block diagram or schematic that demonstrates power to the watchdog feature is not derived from this same regulator function then I believe the standards will be met.

Does anyone have any experience with this kind of thing, or be able to point me in the right direction?

Kind Regards

henry.dick · ‎2018-06-13

Posted on June 13, 2018 at 17:05

I'm not aware of any mcu whose watchdog is separately powered.

So two possibilities:

1. You are reading the standard wrong. Or

2. They have a separate watchdog circuitry.

Chris Reynard · ‎2018-06-19

Posted on June 19, 2018 at 08:39

Thank you for replying.

The standard quotes 'monitoring equipment with its own time base (e.g watchdog) shall work independently and separately from the parts of the digital unit which perform the data processing... Separate operation of the monitoring equipment is considered to be fulfilled if the voltage supply is separately connected to the power supply of the digital unit'.

It also goes onto say 'Independent and separate operation will typically be ensured by using an external monitoring component'.

Cost, power, and ease of use make a built-in watchdog feature very attractive, but may not be considered compliant. Most MCU's have datasheets/documentation that detail the use of an independent clock but lack detail with regards to how they are powered. Hence I was hoping to find something that could demonstrate the above is met.

henry.dick · ‎2018-06-19

Posted on June 19, 2018 at 12:13

here is my take on that.

1. the monitoring equipment needs to have its own time base - not difficult for a typical mcu;

2. the monitoring equipment needs to be independently powered - i have never seen this type of powering scheme;

3. the monitoring equipment is generally external but not all ways - it does provide some leeway for an onboard watchdog but given 2) above, not much leeway.

put that together, I think it is next to impossible to use the watchdog inside of a mcu that's also performing the digital processing to perform the monitoring. if a mcu is indeed used for this purpose, it either has a weird / unusual watchdog power domain, or questionable compliance.

Chris Reynard · ‎2018-06-19

Posted on June 19, 2018 at 12:46

Thanks for the prompt reply. It makes sense what you say and I was beginning to form this opinion also. I know other manufacturers are using in-built watchdogs but it seems questionable practice from a compliancy point of view.

Thanks again.