ESP8266 Community Forum

- Thu Nov 17, 2016 3:36 am #58239 I see a few warnings in the SDK documentation:

We suggest using a timer to check periodically, if users need to call os_delay_us or
function while, or function for in timer callback, please do not occupy CPU more than 15
ms.

Does the latter mean that there is a 15 ms time limit even for software timer callbacks? Why 15 ms?

Read and write RAM has to be aligned by 4 bytes, so please do not cast pointer directly,
for example, please use os_memcpy instead of float temp = *((float*)data);.

Does this apply only if data was some (void *) or (uint8_t *)? Or do they mean that assignment to a dereferenced pointer is invalid? (i.e if data was a float * already, is it still invalid? seem crazy if it is...)

- Thu Nov 17, 2016 10:49 pm #58282 I'm really not sure how valid the 4 byte RAM write/read is. I have something like this

Code: Select alltypedef struct {
   uint16_t size;
   uint16_t source;
   uint16_t destination;
   uint8_t payload[MAX_PAYLOAD_SIZE];
} Data_Packet;

uint8_t buf[100];


I receive the Data_Packet via UART and store it in buf and then I cast the buf into a Data_Packet. And then I cast the payload into the desired data structure. I do this in a few places in my code but my code runs fine most of the time (one crash every few hours). Or is the warning something that may cause problems only occasionally...?

- Fri Nov 18, 2016 12:26 pm #58328

I see a few warnings in the SDK documentation:

We suggest using a timer to check periodically, if users need to call os_delay_us or
function while, or function for in timer callback, please do not occupy CPU more than 15
ms.

Does the latter mean that there is a 15 ms time limit even for software timer callbacks? Why 15 ms?

Yes, it does. The ESP8266 only has a single core and the non-os SDK is only single threaded. The device is not just running your code, it's also running other stuff, like keeping the WiFi association(s) alive. I assume 15ms is, give or take, how frequently it needs to do this.

Read and write RAM has to be aligned by 4 bytes, so please do not cast pointer directly,
for example, please use os_memcpy instead of float temp = *((float*)data);.

Does this apply only if data was some (void *) or (uint8_t *)? Or do they mean that assignment to a dereferenced pointer is invalid? (i.e if data was a float * already, is it still invalid? seem crazy if it is...)

What this is essentially saying is that when accessing some data types, these must be aligned on a 4-byte boundary in RAM. This applies to for example a 4 byte int type. So if you cast an arbitrary piece of memory to an int there a 3 in 4 chance this won't be 4 byte aligned, so the processor will throw an exception.

In reality it may be much less likely, or much more likely, than 3 in 4 depending on the precise code - or it may not happen at all.

You can resolve your problem by declaring your buf to be 4 byte aligned:

Code: Select alluint8_t __attribute__((aligned(4))) buf[100];

esp8266 otb-iot software - https://github.com/piersfinlayson/otb-iot
esp8266 otb-iot documentation - https://otb-iot.readthedocs.io/
esp8266 related blog - http://www.packom.net/