OhmLeds2
| Project OhmLeds2 | |
|---|---|
| Designing successor of Ohmleds | |
| Status | Initializing |
| Contact | cmpxchg |
| Last Update | 2020-06-06 |
Problem description
There is word that ohmleds https://tkkrlab.nl/wiki/OHM_LEDS might not work with all modern switches, due to lack of supporting 10 mbps mode in their PHY or switch fabric. On the other hand, there is demand for new features, like Power-over-ethernet, and a more compact, but still weather-proof design. The OhmLeds design used a microchip ethernet controller with a built-in UDP stack, and SPI link to another microcontroller doing the ArtNet protocol implementation and driving the Leds. A laptop powersupply delivers the power from 230 V. The whole is mounted in a short PVC tube, which is wound around with a RGB LedStrip. The whole is mounted using tie-raps to a datenklo. The artnet protocol https://artisticlicence.com/WebSiteMaster/User%20Guides/art-net.pdf is used to drive and indicate the status of the datenklo.
Exploring solution space
The SPI link between the microcontroller and ethernet controller is unique for the OhmLed 2013 setup. Almost all other microcontrollers use a parallel interface to an ethernet PHY, https://en.wikipedia.org/wiki/Media-independent_interface The idea is to use a microcontroller with such interface.
I would like to explore the space of such solution, since this also allows for PoE (power over ethernet)
Ethernet interfacing
On the PHY side, there is the need for power-over-ethernet. This involves three main components:
- an ethernet transformer that can take off the DC power (20 watts or more)
- a controller for such power-takeoff
- an ethernet PHY that does the actual ethernet analog signal conversion
Ethernet transformers
Coilcraft, wurth..
PoE controller/circuitry
This is a combination of many cheap passives, and a PoE controller. ST has a few options https://www.st.com/content/st_com/en/campaigns/PoE_supply_reference_designs.html
One 8A, 5V solution might be great for driving many Leds, since their forward voltage is typically around 3 volt and are controlled using PWM.
PM8805 4-pair IEEE 802.3bt compliant PD interface (ca EUR 5.00 / 100 pcs) Output stage managed by configurable PM8804 controller (ca EUR 1.05 / 100 pcs) Output voltage: 5 VDC ±2.5% Output current: 8 A
Works with power supplied from Ethernet LAN cables or local auxiliary sources Line input voltage range: 40 to 60 VDC
Peak-to-peak output ripple: <50 mV DC-DC full load efficiency: >91% Overall full load efficiency: >90% Transient response ΔV pk-pk 100% to 50% load step ≈ 700 mV Switching frequency ≈ 280 KHz
Ethernet PHY
There are many solutions, but no offerings from NXP or ST. RealTek, Texas Instruments, Analog Devices, Intel.
This part does the clock recovery, converting via a small (!) fifo to the clock domain and parallel interface of the MII interface of the microcontroller. This part should be reliable, no 'packets of death' should bring this link down, for example, as reported already in februari 2013 in this article, bringing the link down after triggering on seemingly arbitrary bytes at a specific offset in the ethernet frame: https://www.theregister.com/2013/02/06/packet_of_death_intel_ethernet/
Discrete Ethernet PHY
Building a PHY from discrete logic/small FPGA might be explored too. Idea is to explore the usage of an FPGA to build the digital part of the ethernet PHY. 10 megabit ethernet is very simple encoding, but for 100 megabit ethernet, escape sequences are already defined to define framing, and bandwidth limitations due to that frameing.
Mains power
Next to power-over-ethernet, mains power might be needed in PoE is not sufficient or just not working due to vendor-specific implementation details. For this, a discrete 230V to 5V, 40 watt powersupply can be designed on the same, 4-layer board.
LED interface
5 volts, ground, and data/clock soldering pads.