Influence of Core Frequency

[robinho710]

Hi all,

I am not sure if this is the right section for posting this question but it seemed rather advanced for me so I am just gonna ask it here.

I want to use the miniUART of a Raspberry Pi 4b 4GB and have not found too much information about the influence of setting the core frequency to 250 Mhz. Apparently this is automatically done when the miniUART is enabled (https://www.raspberrypi.org/documentati ... on/uart.md) and I was wondering if it has a negative impact on the performance of the pi?

I searched quite a lot on my own but could not find any valuable information in that regard. I found out that the core frequency is (related to) the GPU frequency but not directly related to the CPU frequency. Although I read something, that the core frequency also determines the memory speed.

I am using the Pi headless so limiting the GPU frequency wouldn't be too big of a problem. Limiting the performance of the CPU or memory, however, would be.

I am a little lost so I'd be very happy about a little hint on where to find this information or the information itself

Thanks in advance.

Cheers,
Robin

[cleverca22]

The vpu freq normally changes dynamically based on vpu load, but that messes with the miniuart
Turning it on just locks the vpu at a high frequency I believe

But the core frequency I think is separately set

[robinho710]

Well on the website (https://www.raspberrypi.org/documentati ... on/uart.md) it says:

In order to use the mini UART, you need to configure the Raspberry Pi to use a fixed VPU core clock frequency. This is because the mini UART clock is linked to the VPU core clock, so that when the core clock frequency changes, the UART baud rate will also change. The enable_uart and core_freq settings can be added to config.txt to change the behaviour of the mini UART.

So to me that sounds like the core_freq (core frequency) is the frequency of the VPU core clock. And I would really like to know what impact the throttling has on the overall performance of the PI.

[cleverca22]

So to me that sounds like the core_freq (core frequency) is the frequency of the VPU core clock. And I would really like to know what impact the throttling has on the overall performance of the PI.

i think its less that it throttles the core clock, and more that it just locks it to the fastest speed, and doesnt try to slow down to save power

so your more likely to get better performance, at the cost of more heat and watts used

[jamesh]

And I would really like to know what impact the throttling has on the overall performance of the PI.

In everyday usage, not particularly noticeable.

[robinho710]

So to me that sounds like the core_freq (core frequency) is the frequency of the VPU core clock. And I would really like to know what impact the throttling has on the overall performance of the PI.

i think its less that it throttles the core clock, and more that it just locks it to the fastest speed, and doesnt try to slow down to save power

so your more likely to get better performance, at the cost of more heat and watts used

But I think the core frequency is by default set to 250 Mhz if the miniUART is enabled. Here (https://www.raspberrypi.org/documentati ... locking.md) it says, however, that the default core frequency is 500/550/360. There is also the force_turbo option where I am not sure if it is just used for overclocking. I thought if only enable_uart is set to 1 without the force_turbo, then the core_freq is set to the minimum (250 Mhz) vs. the maximum (400 Mhz) when force_turbo is set to 1 as well.

And I would really like to know what impact the throttling has on the overall performance of the PI.

In everyday usage, not particularly noticeable.

Ok, but it is probably not everyday usage that I want to go for. The Raspberry Pi replaces an old PC104 board in a robot and hence has to communicate motor commands with another board and also run quite demanding algorithms like SLAM. When driving, this does bring the Pi to its limits (all 4 cores fully used). Does this make a difference then?

I am just wondering if there is any other SoC that is better suited for such a use. If someone has experience with that I would greatly appreciate the help.

Thanks for the replies!

[jamesh]

So to me that sounds like the core_freq (core frequency) is the frequency of the VPU core clock. And I would really like to know what impact the throttling has on the overall performance of the PI.

i think its less that it throttles the core clock, and more that it just locks it to the fastest speed, and doesnt try to slow down to save power

so your more likely to get better performance, at the cost of more heat and watts used

But I think the core frequency is by default set to 250 Mhz if the miniUART is enabled. Here (https://www.raspberrypi.org/documentati ... locking.md) it says, however, that the default core frequency is 500/550/360. There is also the force_turbo option where I am not sure if it is just used for overclocking. I thought if only enable_uart is set to 1 without the force_turbo, then the core_freq is set to the minimum (250 Mhz) vs. the maximum (400 Mhz) when force_turbo is set to 1 as well.

And I would really like to know what impact the throttling has on the overall performance of the PI.

In everyday usage, not particularly noticeable.

Ok, but it is probably not everyday usage that I want to go for. The Raspberry Pi replaces an old PC104 board in a robot and hence has to communicate motor commands with another board and also run quite demanding algorithms like SLAM. When driving, this does bring the Pi to its limits (all 4 cores fully used). Does this make a difference then?

I am just wondering if there is any other SoC that is better suited for such a use. If someone has experience with that I would greatly appreciate the help.

Thanks for the replies!

If the ARM's are running full tilt, I doubt changing the VPU will make much difference. Thats just in control of things like the camera and H264 encode/decode and the display output etc, which you probably are not using to any great extent anyway in your application.

Simply overclocking the ARM's will massive outweigh any loses by limiting the VPU frequency. Many people get them up to 2Ghz.