What is EXTIT? (and are there EXTI enable bits for real?)

[waclawek.jan]

Can't post screenshots.

• RM0401 rev3 ('F410), RCC_APB2ENR contains EXTIEN. There is no such bit in any other STM32, EXTI is unconditionally enabled everywhere else. There is no respective reset bit in RCC_APB2RSTR. Is this bit real?
• in the same RM0401 rev3 there's also RCC_APB2LPENR.EXTILPEN, so again, is it real?
• in CMSIS-mandated 'F410 device headers, this bit is marked as RCC_APB2ENR_EXTITEN (note the extra T).
• searching for EXTIT/EXTITEN (with the extra T) in Cube revealed it also in 'F412/'F413/'F423 and in stm32f4xx_hal_rcc_ex.h and stm32f4xx_ll_bus.h
• internet search revealed RM0402 rev.6 ('F412) contains RCC_APB2LPENR.EXTITEN with a rather messy description mentioning "EXTIT APB and SYSCTRL PFREE clock", but no such bit in RCC_APB2ENR (nor RCC_APB2RSTR). So, again is it real?
• RM0430 rev.8 ('F413/'F423) has RCC_APB2ENR.EXTITEN, marked as EXITEN (note missing the other T this time), again referring to "Extit Apb sysctrl pfree clock". There's also RCC_APB2LPENR.EXTITLPEN. Again, are these real?
• RM0438 rev.7 ('L552) contains "EXTIT" (with both Ts) in Table 3. TrustZone-aware peripherals page 86, that's most probably just a typo, shouldn't be there nevertheless.

JW

[Tesla DeLorean]

Other parts had this unit clocked under SYSCFG or AFIO

[waclawek.jan]

> Other parts had this unit clocked under SYSCFG or AFIO

Are you sure?

I can write to EXTI registers in 'F407 without enabling any clock in RCC.

External pins are usually routed to EXTI through a register in SYSCFG, but that does not imply that EXTI itself has its clock gated together with SYSCFG. Do you have an example where this is so?

JW

[Sarra.S]

Hello @waclawek.jan, 

Thank you for these inputs, I'll look into all the issues you raised and get back to you with answers.

[Sarra.S]

Hello again @waclawek.jan, 

>>Is this bit real?

Yes, this bit is mandatory, it has been added to be able to reduce power consumption by gating the EXTI clock. 

nevertheless, it is important to align all the nominations in all reference manuals with the CMSIS (RCC_APB2ENR.EXTITEN and RCC_APB2LPENR.EXTITLPEN)

for that, There are several internal tickets: 

* For RM0430: Ticket 162952 to add EXTITEN bit in "RCC register map and reset values for STM32F413/423"

* For RM0402: Ticket 162957 to modify the bit to EXTITLPEN to be aligned with RM0430, RM0401, CMSIS 

* For RM0401: Ticket 162959 to align EXTIEN with other ref mans and CMSIS  

Thank you for bringing up these issues! 

[waclawek.jan]

Thanks, Sarra. Looking forward to to those changes.

Btw, what does the second "T" stand for in EXTIT?

JW

@Sarra.S 

[Sarra.S]

Actually, EXTIT is the name of the external interrupt controller, it manages up to 32 external interrupt lines and generates the interrupt request to the interrupt controller. 

Well, that was the nomination choice...I'm not sure the T has any significance!

[waclawek.jan]

> Actually, EXTIT is the name of the external interrupt controller

Is it? In STM32 context, I've *never* seen it been called other than EXTI (no T):

That's why I'd strongly suggest to omit the T in the naming of enable bit, too.

JW