How to calculate nRF9160 NVM / flash wear lifetime?

[C.T.]

In the nRF9160 command reference guide it states: "CAUTION: CFUN=0 causes writing to NVM. When using CFUN=0, take NVM
wear into account."

And I've found this post:

 NRF9160 Flash Endurance 

A few questions:

1. Is there anyway to know how many write cycles have been used?

2. If no user parameters were changed with AT commands can we assume AT+CFUN=0 won't use a write cycle?

3. Upon modem registration, is carrier or network information stored in NVM?  At my office, upon very first power up, registration seems to take around 30 seconds. After subsequent power cycles, registration seems to happen pretty quickly. Typically around 4 seconds. Will this impact NVM lifetime?

[C.T.]

Thanks. We will explore if eDRX or PSM is possible for our application and what the power consumption is compared to having the modem powered off between use.

I look forward to hearing the details on how to calculate flash wear lifetime taking into consideration flash use / number of credentials stored in flash.

[Didrik Rokhaug]

Hello again, and sorry for the long wait.

I still haven't gotten a better way to calculate a more average expected lifetime of the modem flash. We are working on it, but it will probably take some time, as there are a lot of different factors that can affect it.

In the meantime, I would like to re-iterate some of my earlier points and questions:

While the expected lifetime will be better than what we have calculated so far in this ticket, it will still not be great as long as you keep using AT+CFUN=0 every 5 minutes.

However, you shouldn't have to use it that often anyway. First of all, if the point of turning off the modem is power saving, the strategy is flawed. Turning the modem back on can consume a lot of power, so unless you are sending very rarely (an interval of several days), PSM and/or eDRX will consume less power. The only other reason for turning the modem off for power saving purposes is if the device is roaming on networks where it might not be granted PSM. However, in this case, you can use AT+CFUN=4 most of the time.

From a power consumption perspective, AT+CFUN=4 and 0 are equivalent (except that AT+CFUN=0 might trigger a flash write, and thus consume more power). The benefit of using AT+CFUN=0 is that network parameters will be stored in flash, which will speed up the network connection when the modem is turned back on. However, this point is only valid if power is disconnected or the modem is fully de-initialized. Otherwise, the network parameters will still be in the modem RAM when you re-enable the modem. In addition, unless the device has moved to a completely different location (one with different networks, i.e. in many cases a different country), even very old network parameters will help speed up the connection. So unless you expect the device to be in a different country every 5 minutes, I don't see any point in using AT+CFUN=0 that often.

Reducing the number of AT+CFUN=0 you issue is also the most effective way of increasing the lifetime of the modem flash. E.g. if you use AT+CFUN=0 only once a day, you will, in the absolute worst case that we have been using so far, get an expected lifetime of at least 27 years.

Best regards,

Didrik

[Didrik Rokhaug]

Hello again, and sorry for the long wait.

I just wanted to inform you that we are mostly finished prducing some numbers for more normal scenarios. We are just doing some final verification, and should have them ready for you early next year.

Have a merry Christmas and a happy new year.

[C.T.]

Thanks for the update. Happy holidays.

[C.T.]

Hi, what's the status on this?