[SAMC] at 50MHz rather than 48MHz - Page 1
Forum section: electronics / general

author: Simon
author_rank: Unknown
timestamp: November 29, 2020, 07:55:00 am
content: I've been looking at clock options on the SAMC series, I notice that in the electrical specifications at 125C the 48MHz RC oscillator can shif to up to 50.4MHz. Hm, so if I was running on a crystal that would stay stable over temperature could I just run at 50MHz?I have no immediate plans to do this I am just looking theoretically at timing applications. With 48MHz each tick is 20.8ns, but at 50MHz each tick is a nice round 20ns, AND this can be practically achieved with a 32.768kHz crystal and the PLL as a frequency of 50MHz less 128Hz can be achived. Obviously with a 32kHz crystal precisely 50MHz can be achieved but you don't get that perfect 1s tick that you can get from the 32.768kHz crystal.

author: ataradov
author_rank: Super Contributor
timestamp: November 29, 2020, 09:04:38 am
content: 48 MHz is not some hard stop number. It is chosen to accommodate all possible variations of clock sources. In reality it will probably work at much faster clocks. I did some overclocking experiments and at room temperature SAM D11 started failing at ~60 MHz. Obviously that was just one sample, and the results are not guaranteed.If you have a clock source that does not drift with temperature, then 50 MHz should work fine.

author: Simon
author_rank: Unknown
timestamp: November 29, 2020, 09:22:14 am
content: yes well seeing that at 125C the RC oscillator is expected to be 50.4MHz to me practically declares the chip to run at 50MHz at 125C. I suppose that although it will run at up to 5V as the CPU is running on 1.2V anyway lowering the supply voltage to 3.3V will help reduce internal heating. What I also wonder, particularly from an EMC perspective is how much of the chip does run at 1.2V, at what point is the level conversion. Is it at the pins or do the peripheries run at the external supply voltage with the level shifting at the interface with the CPU and busses.

author: ataradov
author_rank: Super Contributor
timestamp: November 29, 2020, 09:33:46 am
content: Quote from: Simon on November 29, 2020, 09:22:14 amFigure 7-1. "Power Domain Overview, SAM C20/C21 E/G/J" shows a rough block diagram. Basically the core, the buses, the SRAM, and the digital part of the peripherals all run on 1.2 V. Analog peripherals use the supplied voltage in addition. And of course I/O cells do the final level shifting for the pins.

author: Simon
author_rank: Unknown
timestamp: November 29, 2020, 01:25:01 pm
content: Ah right, I'm surprised it doesn't work to a lower voltage then

author: ataradov
author_rank: Super Contributor
timestamp: November 29, 2020, 07:29:05 pm
content: Again, "works" is a relative term. There is nothing fundamentally different in C21 compared to D21 that "works" down to 1.62V. This is a purely marketing decision. This part was not destined to be a low voltage part.There is even less difference between SAM D21 and SAM DA1, they are described by the same datasheet. Yet the first one is 1.62 V part, and the other one is 2.7V min.It is all about what manufacturer chooses to characterize and guarantee those characteristics.

author: Simon
author_rank: Unknown
timestamp: November 29, 2020, 09:59:25 pm
content: Well I have to stay within the datasheet as nothing warrants the risk but if they are that tolerant to lower voltage then 2.7V is definitely viable as a supply voltage with some 2.7 and 2.8V regulators available.Has anyone ever just powered the 1.2V core from a 1.2V regulator into the bypass capacitor pin? seems silly that 75% the power into the logic chip is actually burnt off in an internal regulator, although I guess it is not much with any power from the pins coming from the main rail anyway but starts to make sense why the outputs are only 2.5mA, you can't just power an LED off a pin to see if it works..

author: ataradov
author_rank: Super Contributor
timestamp: November 29, 2020, 10:10:29 pm
content: Powering the core directly is definitely not supported and will not disable the internal regulator, you will just fight it. It is possible to do that, I did it to recover locked devices. But that is not something I would do for devices I care about.For normal pins the worst case current is at 2.7 V is 3 mA for output high with DRVSTR=1. This is plenty to power indication LEDs. I mean the eval kit has on-board LED that works fine.

author: Simon
author_rank: Unknown
timestamp: November 30, 2020, 10:07:42 am
content: Yea i guess that modern LED's on a dev board don't need much current but I'd not use it on a product like i may have in the past.

author: ataradov
author_rank: Super Contributor
timestamp: November 30, 2020, 05:24:25 pm
content: You can connect anode to VCC and cathode to the pin. Current sinking capabilities are higher.And even at 3 mA modern LEDs are blindingly bright.I don't see that as a concern at all, I see no reason to avoid direct connection. But if you want to place extra components for no real reason, it is fine too.

author: poorchava
author_rank: Super Contributor
timestamp: November 30, 2020, 06:47:22 pm
content: The clock rating is just to accommodate for all peripherals, entire temperature range, manufacturing deviations and so on.I recall like 10 or so years ago, some PIC32MX parts could be overclocked by about 100% and still worked in principle. Just some more analogy things like ADC, I2C and so on didn't work.I also overclocked the STM32H7 recently from nominal 480 MHz and as long as I ran it from RAM, it was stable at 540Mhz.On the other hand, i recall one project that used a SAM4S, and when I fouled up PLL settings and the clock went up from nominal 120MHz to 150, it would lock up so hard trying to boot, that I couldn't even connect a debugger to erase it. Thankfully those chips have a dedicated Erase pin.

author: poorchava
author_rank: Super Contributor
timestamp: November 30, 2020, 06:54:35 pm
content: Quote from: Simon on November 29, 2020, 09:59:25 pmI did that with SAM4S. Ran it from a ultra high efficiency dc/dc (TPS62120 if my memory serves me right). The thing has been commercially in production for 3 years now (about 2k units made) and it is 100% ok. The not so nice part is, that thy recommend filtering separately 3v3, 3v3a, 1v2core and 1v2pll, so you end up with 4 supplies each with its own filtering under a QFP100. Thankfully the board was 4L anyway due to other reasons.This mode of operation is actually mentioned somewhere in some reference documentation - it's not a hack or anything.

author: ataradov
author_rank: Super Contributor
timestamp: November 30, 2020, 07:35:46 pm
content: SAM4S is explicitly designed to work with the external voltage regulator. SAM C21 is not designed for that. There is no separate voltage regulator output that you can just leave disconnected if internal regulator is not used. The only reason you have access to VDDCORE is to place a bypass capacitor there, nothing more.