How to implement 512 bytes characteristic in peripheral

RE: How to implement 512 bytes characteristic in peripheral

Duoran Li — Sun, 22 Jan 2017 07:10:59 GMT

what do you change？It reads more data than what one data packet can hold.

RE: How to implement 512 bytes characteristic in peripheral

Paul — Fri, 27 May 2016 16:15:02 GMT

OK, I figured it out. Now I know what does "read long" do. It reads more data than what one data packet can hold. It will send AttReadBlob requests.

RE: How to implement 512 bytes characteristic in peripheral

Paul — Fri, 27 May 2016 16:05:41 GMT

I am doing some test to verify this now. I have a characteristic set to init_len to 40, and max_len = 40. But on Master Control Panel, it only reads the first 20 bytes when I press "discover services" button. When I press "Read" button in the log window it only shows 20 bytes too. I want to verify I can put more than 20 bytes out before I test my device with Apple devices.

RE: How to implement 512 bytes characteristic in peripheral

Paul — Sat, 14 May 2016 01:39:36 GMT

Thanks guys, now I "almost totally" understand how it works by your answers.

RE: How to implement 512 bytes characteristic in peripheral

RK — Sat, 14 May 2016 00:38:43 GMT

No the central is handling the multi byte packet and offsetting logic by sending repeated ATT calls with different offsets in them. The softdevice just sends back the bytes at that offset, up to the max MTU, or an error message if the characteristic doesn't have bytes past that offset.

RE: How to implement 512 bytes characteristic in peripheral

Paul — Fri, 13 May 2016 15:42:41 GMT

Sweet, I thought I saw your similar comment somewhere else. LoL. So the softdevice is handling this multiple packet and offsetting logic?

RE: How to implement 512 bytes characteristic in peripheral

RK — Fri, 13 May 2016 12:29:24 GMT

Central doesn't know nor care whether the property is fixed or variable length, it's not something reported as a property of the characteristic. The central, when reading, will just read at ever larger offsets until it gets a shorter than MTU packet or a response saying there is no data at that offset.

RE: How to implement 512 bytes characteristic in peripheral

Paul — Fri, 13 May 2016 12:18:53 GMT

Thanks Martin. Is attribute length to be variable or fixed defined in the central? The spec I have for the peripheral only says length up to 512 bytes. Basically I have to use variable attribute length. But does central care? Or central is going to detect this vlen bit to decide what to do.

RE: How to implement 512 bytes characteristic in peripheral

MartinBL — Fri, 13 May 2016 07:30:43 GMT

When you initialize your characteristic then write:

attr_md.vlen       = 1; // <- variable attribute length
attr_md.vlen       = 0; // <- fixed attribute length

ble_gatts_attr_md_t Struct Reference

RE: How to implement 512 bytes characteristic in peripheral

Paul — Thu, 12 May 2016 13:25:39 GMT

I am testing the code right now. How do I define the attribute value to be fixed or variable? Thanks.

RE: How to implement 512 bytes characteristic in peripheral

Paul — Thu, 12 May 2016 11:37:21 GMT

Thanks RK! I am writing a peripheral. You made my day :-)

RE: How to implement 512 bytes characteristic in peripheral

RK — Thu, 12 May 2016 03:08:36 GMT

The 20 byte limitation is for notifications and indications. Characteristics can be as long as the stack supports. If you're writing the peripheral side (which I assumed without asking, sorry if you're not), you have nothing to do, central implementations (eg iOS, android) do the necessary repeated reads to get the whole characteristic, if what they read is a full MTU, they just read the next bit until they get something less than a full MTU.

If you're writing the central side then yes you have to do the repeated reads. There's a message sequence chart for that but I don't think there's an example, I can't find one in which the gattc read function is called with anything other than 0 offset, because all the examples use services which have characteristics with fixed lengths shorter than MTU.

Is this peripheral or central code?

RE: How to implement 512 bytes characteristic in peripheral

Paul — Thu, 12 May 2016 02:57:04 GMT

I thought there is a limitation every packet is only 20 bytes and you need to write your own code to use offsets to let all 512 bytes be read.

RE: How to implement 512 bytes characteristic in peripheral

RK — Thu, 12 May 2016 02:33:58 GMT

Example? Just set the length of the characteristic to what you want when you add it.

ble_gatts_attr_t.max_len
ble_gatts_attr_t.init_len

and note the defines

#define BLE_GATTS_FIX_ATTR_LEN_MAX (510)  /**< Maximum length for fixed length Attribute Values. */
#define BLE_GATTS_VAR_ATTR_LEN_MAX (512)  /**< Maximum length for variable length Attribute Values. */