When a software update is requested by an OTAP Client (a device that receives a software update, commonly Bluetooth LE Peripheral) from the OTAP Server (a device that sends a software update, commonly Bluetooth LE Central), you may want to preserve some data previously acquired, such as bonding information, trimming values for the system oscillators, or probably NVM data for your application. This document guides you in performing OTAP updates preserving the flash data content of your interest. This document is intended for developers familiarized with OTAP custom Bluetooth LE service, for more information, you can take a look at the following post: Reprogramming a KW36 device using the OTAP Client Software.
OTAP Protocol implements a format for the software update that is composed of a header and a defined number of sub-elements. The OTAP Header describes general information about the software update and it has a defined format shown in the following figure. For more information about the header fields, you can go to 11.4.1 Bluetooth Low Energy OTAP header chapter of the Bluetooth Low Energy Application Developer's Guide document included in the SDK at <SDK_2.2.X_FRDM-KW36_Download_Path>\docs\wireless\Bluetooth
Each Sub-element contains information for a specific purpose. You could implement your proprietary fields for your application (For more information about sub-element fields, you can go to 11.4.1 Bluetooth Low Energy OTAP header chapter of the Bluetooth Low Energy Application Developer's Guide document included in the SDK at <SDK_2.2.X_FRDM-KW36_Download_Path>\docs\wireless\Bluetooth).
OTAP includes the following sub-elements:
|Image File Sub-element||Value Field Lenght (bytes)||Description|
|Upgrade Image||Variable||This sub-element contains the actual binary executable image which is copied into the flash memory of the OTAP Client device. The maximum size of this sub-element depends on the target hardware.|
|Sector Bitmap||32||This sub-element contains a sector bitmap of the flash memory of the target device which tells the bootloader which sectors should be overwritten and which leave intact. The format of this field is the least-significant bit first for each byte with the least significant bytes and bits standing for the lowest memory sections of the flash.|
|Image File CRC||2||This is a 16-bit CRC calculated over all elements of the image file except this field itself. This element must be the last sub-element in an image file sent over the air.|
The KW36 Flash is partitioned into:
The Bitmap sub-element is 256 bits of length, in terms of the KW36 flash, each bit represents a 2KB sector covering the address range from 0x0 - 0x0007_FFFF (P-Flash to FlexNVM Alias address range), where 1 means that such sector should be erased and 0 means that such sector should be preserved. The Bitmap field is used by the OTAP Bootloader to obtain the address range which should be erased before programming the KW36 with the software update, so it must be configured before sending a software update to leave intact the address range of memory that contain data of your interest and erase only the address range that will be overwritten by the software update.
For example: Suppose that a developer wants to preserve the address range between 0x7D800 - 0x7FFFF and the address range between 0x0 - 0x1FFF, and the left memory must be erased. The address range between 0x7D800 - 0x7FFFF corresponds to the 5 top flash sectors and the address range between 0x0 - 0x1FFF is the lowest 4 sectors.
So, it means that bits between 256 and 252 (256, 255, 254, 253 and 252) and bits between 4 and 1 (4,3,2 and 1) should be set to 0, that way OTAP Bitmap for this example is: