Timer speed/frequency STM32F411 - STM32F407

Posted on May 29, 2015 at 21:04

It does seem a bit odd, but I suspect that 100 MHz max is correct for all the timers.

The first part of the Reference manual excerpt is fairly clear, max upstream clock is 100 MHz, if the APB prescaler is 1 then 100 / 1 = 100.  If an APB prescaler is 2 (e.g., for APB1 @ 50 MHz), you could have 100 / 2 x 2 = 100.

The TIMPRE bit description below is less clear, but I think it's safe to conclude that 100 MHz is still the maximum timer clock rate.

 

Reference manual excerpt,

''The timer clock frequencies for STM32F411xC/E are automatically set by hardware. There are two cases:

1. If the APB prescaler is 1, the timer clock frequencies are set to the same frequency as

that of the APB domain to which the timers are connected.

2. Otherwise, they are set to twice (×2) the frequency of the APB domain to which the

timers are connected.

The timer clock frequencies are automatically set by hardware. There are two cases

depending on the value of TIMPRE bit in RCC_DCKCFGR register:

• If TIMPRE bit is reset:

If the APB prescaler is configured to a division factor of 1, the timer clock frequencies

(TIMxCLK) are set to HCLK. Otherwise, the timer clock frequencies are twice the

frequency of the APB domain to which the timers are connected: TIMxCLK = 2xPCLKx.

• If TIMPRE bit is set:

If the APB prescaler is configured to a division factor of 1 or 2, the timer clock

frequencies (TIMxCLK) are set to HCLK. Otherwise, the timer clock frequencies is four

times the frequency of the APB domain to which the timers are connected: TIMxCLK =

4xPCLKx.''

Posted on May 29, 2015 at 21:17

All the STM32 designs allow for the timers to pull their clock from one-tap earlier on the divider chain, except the DIV1 case where there isn't a faster source. Review the ''Clock Tree'' diagram in the Reference Manual for each series of parts.

The 407 has APB1 at DIV4, ABP2 at DIV2, APB2 here being nominally 84 MHz and the TIMCLKs attached at 168 MHz

The 411 has APB1 at DIV2, ABP1 at DIV1, the TIMCLKs on both these buses can be 100 MHz.

The general philosophy at work here is that any counter/timer is going to at least divide it's input by TWO. ie one clock tick up, one clock tick down.

Posted on June 16, 2015 at 18:45

Thank you for your answer. It sounds very interesting, but I must admit, that I did not understand it really.

Going from Discovery STM32F407 to Discovery STM32F411, if I want to operate the Discovery 411 with 84 MHz for simplicity, I understand that I should do the following changes in the clock configuration file system_stm32f4xx.c:

Parameter       Discovery407     Discovery411   Remark

PLL_P            2                        4                     to reduce core speed to 84MHz

RCC_CFGR

                     RCC_CFGR_PPRE2_DIV2 | RCC_CFGR_PPRE1_DIV4

                                             RCC_CFGR_PPRE2_DIV1 | RCC_CFGR_PPRE1_DIV2

                                                                        to set ABP1 to 42MHz and ABP2 to 84MHz

FLASH_ACR

                    FLASH_ACR_LATENCY_5WS

                                            FLASH_ACR_LATENCY_2WS

                                                                        for 84MHz 2 WS required (RM 3.4.1 Table 5)

PWR_CR    PWR_CR_VOS  PWR_CR_VOS*2  see RM3.4.1 Table5

I hope this is correct? (Or do your recommend any further changes?)

Could you specify the settings for 100MHz for 411? I have no glue to get this accomplished, as in RM-411-6.3.3 (description of register RCC_CFGR) it is clearly stated for the Parameters PPRE2 and PPRE1, that APB2 and APB1 must not run at higher speed than 84MHz/42MHz. I have no idea how I should accomplish this with 100MHz core frequency (this also collides with the 411 datasheet spec in the timer table (DS 411 3.20 Table4), where the max interface clock is specified with 100MHz - but I assume with interface clock they mean the speed of APB2 and ABP1??????)

PS: Looking through this, I recognized that the Prefetch is not enabled in my FLASH_ACR configuration (I still always used the system_stm32f4xx.c from the discovery project - it is a pity that this discovery project is somehow not available for the STM32F411). Can you give me an estimation, how much % a typical C program will run higher in speed, if I enable prefetch by setting the PRFTEN bit? (or is this not really recommended?).

Posted on June 16, 2015 at 19:19

I think any references to max 84/42 MHz are due to a failure to completely edit the document. Most of the materials, and peripheral speeds quoted (SPI, USART, TIM) are predicated on 100/50 MHz clocks.

''Several prescalers are used to configure the AHB frequency, the high-speed APB (APB2) and the low-speed APB (

APB1

) domains. The maximum frequency of the AHB domain is 100 MHz. The maximum allowed frequency of the high-speed APB2 domain is 100 MHz.The maximum allowed frequency of the low-speed APB1 domain is 50 MHz''

http://www.st.com/web/en/resource/technical/document/reference_manual/DM00119316.pdf

Getting to 48 MHz might be problematic. For USB consider 96/48 MHz

For 100 MHz (VOS and Flash wait states set appropriately)

PLL_M = 8 (8 MHz source to 1 MHz comparison)

PLL_N = 200

PLL_P = 2

APB1 DIV2

APB2 DIV1

or

PLL_N = 400

PLL_P = 4

Comparison frequency between 1-2 MHz, VCO frequency between 192 and 432 MHz

Posted on June 16, 2015 at 19:37

Can you give me an estimation, how much % a typical C program will run higher in speed, if I enable prefetch by setting the PRFTEN bit? (or is this not really recommended?).

At 100 MHz, code that would take 100 cycles, would take 60 cycles with prefetching enabled, as I read the documentation. One could presumably benchmark.

The ART is quite effective at hiding the inherent slowness of the flash array.

Posted on June 16, 2015 at 19:48

Thanks, very helpful.

Now I just also resolved another mystery which I did not get before:

- This TIMPRE bit, in fact the complete RCC_DCKCFGR register is brand new as well in STM32F411 as in the STM32F42x/43x parts ... . So in STM32F411 all timers can run at 100MHz if this bit is enabled, and in STM32F42x/43x all timers can run at 168MHz if this bit is enabled.

... it would be really nice, if STM could invest in an application note which gives a table of register differences for the different parts of the STM32F4 family ... (only for the basic processor blocks as RCC, ...). This would be really very helpful.

Posted on June 16, 2015 at 21:44

... just you over-estimated the prefetch influence quite a bit.

In my software it brings a speed increase of only 10% - but more than nothing of course ... .

(STM32F407 discovery, 168MHz, 5 Wait States). I did not try with STM32F411 yet.

Posted on June 17, 2015 at 08:28

Adapting the code to the ART would not really be realistic :). It is just a complex C software which does some very complex, user programmable LED blinking now for testing purpose ... .

If I compare the cycle time to the slowest case of all switched off, I get the following timings:

(all with STM32F407, 168MHz, VOS set, 5 WS):

all off: 104usec

DCEN: 100usec (-4%)

PRFTEN: 92usec (-11.5%)

DC+PRFTEN: 89usec (-15%)

ICEN: 85usec (-19%)

ICEN+DCEN: 82usec (-22%)

ICEN+PRFTEN: 77usec (-26%)

ICEN+DCEN+PRFTEN: 73usec (-30%)

So the most important seems to be the Instruction cache, then the Prefetch, then the Data cache ... (that DC is not too important is understandable, my software does not use large Data table handling ...).

... just info ...