SparkFun RED-V Thing Plus - SiFive RISC-V FE310 SoC

[netmonk]

I just bought two of them on mouser, after playing with longan nano for a few months now.

I would like to know what is the use of the mk22fn128 https://www.nxp.com/products/processors-and-microcontrollers/arm-microcontrollers/general-purpose-mcus/k-series-cortex-m4/k2x-usb/kinetis-k22-120-mhz-cost-effective-full-speed-usb-microcontrollers-mcus-based-on-arm-cortex-m4-core:K22_120  on it ?
Just to deal with usb connexion ?

[thm_w]

https://www.sparkfun.com/products/15799
https://cdn.sparkfun.com/assets/a/c/3/e/4/RedVThingPlus.pdf

The RED-V Thing Plus comes programmed with a simple bootloader The modern USB-C connector makes it easy to program over USB connectivity or use as a JTAG interface via the FE310's NXP K22 ARM Cortex-M4. For more advanced users who prefer to use the power and speed of professional tools, we've also exposed the JTAG connector.

They are using it as a USB->JTAG interface, to be able to program and debug the FE310. It looks like a serial connection is there as well.

FE310 doesn't appear to have any sort of USB interface: https://groupgets.com/campaigns/462-freedom-e310-fe310-mcu

[netmonk]

Interesting, why such an mcu to act like a usb-serial interface ?

[thm_w]

Not sure.
Maybe they use it elsewhere, or already had USB JTAG code provided that would work on it, and need decent jtag performance?

[SiliconWizard]

As said above, it's used both for JTAG and a general purpose serial interface.
Obviously they already have working code for this MCU. The board has a compatibility layer for the Arduino environment.
Yes Sparkfun likely uses this part already on a number of other boards.

I don't see what exactly you'd find weird. You get this USB connectivity with JTAG+serial, and being an MCU, it's customizable enough to easily interface it with the Arduino IDE, and is just a few bucks. What's the problem?

[PCB.Wiz]

Interesting, why such an mcu to act like a usb-serial interface ?

Yes,  what they have chosen is neither small or very cheap, and seems to be only FS-USB speed too.
As above, likely they already have code for the Freescale part, as it is mature. They may also have 'lots in stock' ....

[netmonk]

well what i find bizare is that this mcu seems as much powerfull than the risc mcu.
And beside just using it as a usb/jtag bridge, it cannot be much used for other purposes.

[SiliconWizard]

well what i find bizare is that this mcu seems as much powerfull than the risc mcu.
And beside just using it as a usb/jtag bridge, it cannot be much used for other purposes.

It's relatively cheap by quantities.
Who cares if it's more powerful than the FE310? The board needed a JTAG interface, and it was likely (given their context) one of the cheapest ways to provide that while being flexible enough.

In a similar idea, there are also a number of dev boards embedding a relatively powerful MCU to implement a licensed Segger J-link probe. Sometimes this MCU is "more powerful" than the evaluated MCU. What's the problem? It's just a tool.

What would you have used instead?

[netmonk]

I was just questionning the underlying logic to use such a powerfull side mcu just for jtag and usb bridge.

As im pretty new to mcu, and mainly focused on esp32 board, i was accustomed to meet such CP2102/9 usb bridge, to do the job (not jtag it seems).

But i was not aware of such practice to have a powerfull side mcu for usb/jtag bridging such as in this sparkfun board. i found it a little strange.

Excuse my noobiness.

[SiliconWizard]

A simple serial channel would typically only allow uploading code to the target MCU, or program Flash... provided that said MCU has a serial bootloader. You usually can't use a debugger with just serial communication.

AFAIK, the FE310 doesn't have a serial bootloader, so that wouldn't be workable (someone correct me if I missed it). Besides, with JTAG access you get access to a debugger, and can do a lot more.
So, here you have your explanation why they chose this approach. (The ESP32 has a serial bootloader in its internal ROM.)

As far as cost goes, for a dev board, it's really not a big deal. In quantities (at least through usual distributors), a CP2102 would be like ~$2 cheaper than the MCU they used on this board (but anyway, as I said, a CP2102 wouldn't be enough in this case.) So it's not as big a deal as you may imagine.

[netmonk]

Thank you so far for your explanations. it's far more clear now.

Im currently working an a drone firwmare esp32 based, i was curious to shift to riscV mcu, that's why i was questionning this sparkfun.

I have two of them, time to play with them.

[errorprone]

This microcontroller is a standard one for the ARM Daplink debugger and is used on a lot of Kinetis FRDM development boards.  Probably easier to repurpose than to start fresh.  The project is open source https://github.com/ARMmbed/DAPLink

[PCB.Wiz]

well what i find bizare is that this mcu seems as much powerfull than the risc mcu.
And beside just using it as a usb/jtag bridge, it cannot be much used for other purposes.

It may seem strange at first glance, but it is actually becoming more common.
I like to see a Debug Bridge part with a HS-USB link, as that makes the debug experience more responsive, and I think the newest ATTiny boards used a higher end Atmel MCU, that is far more powerful than the 8-bit part sat next to it.
Likewise, the newest Nuvoton 8 bit MCU Eval boards have a HS-USB Arm 4 MCU as the bridge.
Usually these better bridge parts also support a Serial COM port as well as the Debug-link.