| Electronics > Beginners |
| Creating controller for LED display, help / review needed |
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| Lt_Flash:
Sorry, my mistake, you're doing a thermal relief connection style for polygons, in that case - no need to cover pads with polygon as it will produce three thinner connections instead of your beefy one! I just so got used to have direct connect style that I'm making such stupid mistakes :) |
| Lt_Flash:
And another though - because you've got such a big 3V3 polygon and just a single connection from regulator - you may want to put some beefy tantalum cap somewhere on 3V3 rail, something like 47uF or 22uF or at least 10uF - that would help to keep 3V3 steady in case when fast switches of levels are generated by MCU, for example and would smooth output ripple of regulator. I'm not sure how much current would your board draw but usually it's a good idea to have such cap on 3V3 or any other power rail along with bypass cap (100nF - 1uF). |
| Lt_Flash:
Final thought, I'm not sure if you have checked this document yet, but here's a full design documentation for SAMD51/E54 Xplained Pro board, I'd choose same decoupling caps on all the pins of the MCU to guarantee it's stable performance. I'm not saying your design is incorrect, but maybe you just want to check it against a real production board that uses such MCU. That's what I usually do in my designs. Hope this helps! http://ww1.microchip.com/downloads/en/DeviceDoc/SAM_E54_Xplained_Pro_Design_Documentation.zip |
| Lt_Flash:
Just out of curiosity - why did you choose such a powerful and complex MCU? Do you really need it? Because there are models like SAMD21, SAML21 in a 32-pin package and with 48MHz and full-speed USB onboard, or even in a SOIC-14 package, if you don't need to many GPIO ports. Easier to do a PCB for these ones, easier to debug and with almost all features. FPU is great and 120MHz too, but as far as I understand - you just need to talk to RS485 bus and to display, I2C and SPI buses? All that can be achieved with SAMD2x/L2x MCUs without any trouble. Also, external crystals are only required if you really want a high-precision stability, from my experience I had no issues to use internal oscillators of these MCUs, didn't notice any significant (to my applications, including FreeRTOS) time drift, but they allow to save a lot of power if you're operating from a battery or switched into a battery backup mode. SAML-series MCUs are the best for lowest power applications. In regards to external crystals - you also want traces to them to be as short as possible and of equal length for best stability. |
| muktupavels:
--- Quote from: Lt_Flash on July 28, 2018, 03:17:39 pm ---In regards to GND vias - usually you would want to have them as that shortens your current return path, thus reducing impedance of a trace. --- End quote --- Thanks! --- Quote from: Lt_Flash on July 28, 2018, 03:28:21 pm ---And another though - because you've got such a big 3V3 polygon and just a single connection from regulator - you may want to put some beefy tantalum cap somewhere on 3V3 rail, something like 47uF or 22uF or at least 10uF - that would help to keep 3V3 steady in case when fast switches of levels are generated by MCU, for example and would smooth output ripple of regulator. I'm not sure how much current would your board draw but usually it's a good idea to have such cap on 3V3 or any other power rail along with bypass cap (100nF - 1uF). --- End quote --- I think I only have two electrolytic caps - 10uf and 100uf: - https://www.digikey.lv/product-detail/en/panasonic-electronic-components/EEA-GA1C100B/P15786CT-ND/3831153 - https://www.digikey.lv/product-detail/en/panasonic-electronic-components/EEA-GA1C101B/P15796CT-ND/3831163 Will 100uf be good enough or I really need tantalum? --- Quote from: Lt_Flash on July 28, 2018, 03:33:53 pm ---Final thought, I'm not sure if you have checked this document yet, but here's a full design documentation for SAMD51/E54 Xplained Pro board, I'd choose same decoupling caps on all the pins of the MCU to guarantee it's stable performance. I'm not saying your design is incorrect, but maybe you just want to check it against a real production board that uses such MCU. That's what I usually do in my designs. Hope this helps! http://ww1.microchip.com/downloads/en/DeviceDoc/SAM_E54_Xplained_Pro_Design_Documentation.zip --- End quote --- All pins? I think I have decoupling cap on every power pin, did I miss something? I have seen that and I have pdf open still... That board use 128 pin device, but I have 64 pin... --- Quote from: Lt_Flash on July 28, 2018, 03:47:04 pm ---Just out of curiosity - why did you choose such a powerful and complex MCU? Do you really need it? Because there are models like SAMD21, SAML21 in a 32-pin package and with 48MHz and full-speed USB onboard, or even in a SOIC-14 package, if you don't need to many GPIO ports. Easier to do a PCB for these ones, easier to debug and with almost all features. FPU is great and 120MHz too, but as far as I understand - you just need to talk to RS485 bus and to display, I2C and SPI buses? All that can be achieved with SAMD2x/L2x MCUs without any trouble. Also, external crystals are only required if you really want a high-precision stability, from my experience I had no issues to use internal oscillators of these MCUs, didn't notice any significant (to my applications, including FreeRTOS) time drift, but they allow to save a lot of power if you're operating from a battery or switched into a battery backup mode. SAML-series MCUs are the best for lowest power applications. In regards to external crystals - you also want traces to them to be as short as possible and of equal length for best stability. --- End quote --- 32 MHz xmega was not fast enough. Text was scrolling too "slow", I was hoping that I could workaround that by moving content by multiple positions at one time, but that had side-effect - text suddenly looks very bold... I don't want to redo pcb if internal oscillator will not be good enough. That pcb will be outside of building and has to work in winter and/or summer and from what I have read temperature can affect stability. |
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