关于NVIC的分组

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在Cortex™-M3 Technical Reference Manual中的P166关于中断的分组如下：
PRIGROUP    Split of pre-emption priority from subpriority
0                  7.1 indicates seven bits of pre-emption priority, one bit of subpriority
1                  6.2 indicates six bits of pre-emption priority, two bits of subpriority
2                  5.3 indicates five bits of pre-emption priority, three bits of subpriority
3                  4.4 indicates four bits of pre-emption priority, four bits of subpriority
4                  3.5 indicates three bits of pre-emption priority, five bits of subpriority
5                  2.6 indicates two bits of pre-emption priority, six bits of subpriority
6                  1.7 indicates one bit of pre-emption priority, seven bits of subpriority
7                  0.8 indicates no pre-emption priority, eight bits of subpriority.

在PM0056 Programming manual STM32F10xxx/20xxx/21xxx/L1xxxx
Cortex®-M3 programming manual中的P135对中断的分组如下

为何两者的说法不统一，前者PRIGROUP范围0-7，后者PRIGROUP只列了3-7，当PRIGROUP为3时，前者的subpriority由4位表示（16个），后者的subpriority没有，如何解释呢？

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