在做基于K81的项目开发。因为要用到K81的安全存储，特咨询相关技术信息。目前方案是密钥密文信息存储在secure SRAM，要求外部断电之后，可以继续保存信息。见官方文档，secure SRAM在VLLSx或复位之后，都会擦除信息，在LLS模式下可以保持。在此种情况下，如何保持外部电源断开之后，设备仍可保持信息呢？有没有范例方案呢？是否需要内置电池以维持K81工作在LLS模式？
There are two kinds of security storage in K81， as below：
1. VBAT register file (128bit + 32B)
2. secure SRAM (2KB)
the diff between 1 and 2 as below:
VBAT register file also is called battery backup area, VBAT supply power for battery backup area. so please connect coin battery to VBAT pin. but VCC3.3 supply power for secure SRAM.
1. Upon your requirement, we suggest to use battery backup area instead of secure SRAM.
2. If security data are more than battery backup area, suggest that main key is stores in battery backup area, the others encrypted by main key are stored in flash.
3. Suggest that temporary security or private data are stored in secure SRAM.
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