Friday, November 1, 2013

Managed DC PDU

As I mentioned in my last post, powering all of the prototyping boards on my desk presents some unique challenges. With only one exception (the Xilinx AC701 board), each of the 22 boards requires 5VDC at somewhere between 0.1 and 2 amps. Some are strictly USB powered, some have a 5.5/2.1mm barrel jack, and some can be powered by either USB or a barrel jack.

Powered USB hubs would reduce the number of power sources required, so I did just that. Lots of cables would get in the way so I designed a custom "backplane" USB hub with male mini-B ports which could plug directly into small prototyping boards. (As a side note, the connectors for this board were nearly impossible to find. There are very few uses for a male mini-B connector that mounts to a PCB rather than being attached to a cable so nobody makes them!)

USB backplane hub
These reduced the problem, but did not come close to eliminating it. I still had to power three backplane hubs, six standalone FPGA boards, and four standalone MCU/SoC dev boards. All needed 5V except for the AC701 (which runs on 12V) but I wanted additional 12V capability for the future if I expanded into higher-power design.

The obvious first idea was an ATX supply. My calculations of peak power for the apparatus (including room for growth) were fairly high, though, and most ATX supplies put the bulk of their output on the 12V rail and have fairly limited (well under 100W) 5V capacity.

The next thing I considered was an off-the-shelf 5V supply. This looked like a nice idea, but (as with an ATX supply) the high output current capability would represent a fire hazard if something shorted. I would obviously need overcurrent protection.

Thinking a bit more, I realized that fusing was probably not the best option. Fuses need to be replaced once blown and in a lab environment overcurrent events happen fairly often. Classical current limiting techniques would be problematic as well since many of my boards have switching power supplies. Since a switcher is a nonlinear load, reducing the input voltage doesn't actually reduce the current. Instead, load current actually increases to maintain the output voltage, which can lead to runaway failure conditions. The safer way to handle overcurrent on a switcher is to shut it down entirely.

I also wanted the ability to power cycle boards on command to reset a stuck board or test power-up behavior. While jiggling cables may work in a hands-on lab environment, it isn't a viable option in the remote-controlled "embedded cloud" platform I'm trying to build.

This would obviously require some intelligence on the part of the power management system. The natural solution was a managed power distribution unit (PDU) of the sort commonly used in datacenters for feeding power to racks of servers. Managed PDUs often include current metering as well, which could be very useful to me when trying to minimize power consumption in a design.

There's just one problem: As far as I can tell, nobody makes managed PDUs for 5V loads. The only ones I saw were for 12/24/48V supplies and massively overpriced: this 8-channel 12V unit costs a whopping $1,757.

What to do? Build one myself, of course!

The first step was to come up with the requirements:
  • Remote control via SNMP
  • Ten DC outputs fed by external supply
  • 4A max load for any single channel, 20A max for entire board
  • Independent overcurrent shutdown for each channel with adjustable threshold
  • Inrush timers for overcurrent shutdown to prevent false positives during powerup
  • Remote switching
  • Current metering
  • Thermal shutdown
  • Under/overvoltage shutdown
  • Input reverse voltage protection
  • Able to operate at 5V or 12V (jumper selected)
Now that I had a good idea of what I was building, it was time to start the actual design. I decided to use an FPGA instead of a MCU since the parallel nature made it easy to meet the hard-realtime demands of the overcurrent protection system. I also wanted an opportunity to field-test my softcore gigabit-Ethernet MAC, one of my CPU designs, and several other components of my thesis architecture under real-world load conditions.

PDU block diagram

The output stage is key to the entire circuit so it was very important that it be designed correctly. I put quite a bit of effort into component selection here... perhaps a bit too much, as I missed a few bugs elsewhere on the board! More on that later.

Output stage
Working from the output terminal (right side, VOUT_1) we first encounter a a 5 mΩ 4-terminal shunt resistor which feeds the overcurrent shutdown circuit and current metering. This is followed by a an LC filter to smooth the output power and reduce coupling of noise between downstream devices.

The fuse is provided purely as a second line of defense in the event that the soft overcurrent protection fails. As a firmware/HDL developer I know all too well what bugs are capable of, so I like to include passive safeguards whenever reasonably possible. Assuming that my code works correctly, this fuse should never blow even if the output of the PDU was connected to a dead short. (This of course requires that my protection mechanism trip faster than the fuse. Given the 1ms response time of typical fuses to small overcurrents, this isn't a very difficult task.)

Power switching is done by a high-side P-channel MOSFET connected to VOUT (the main high-current power rail). The logic-level input from the control subsystem is shifted up to VOUT level by an N-channel MOSFET. A pullup and pulldown resistor ensure that the output is kept safely in the "off" state when the system is booting.

Current monitoring
The monitoring stage is even simpler: the shunt voltage is amplified by a TI INA199A2 instrumentation amplifier, then fed to an ADC (not shown in this schematic) for metering. A comparator checks the amplified voltage against a reference voltage set by a DAC (also not shown) and if the threshold is exceeded the overcurrent alarm output is asserted.

A module in the FPGA controls the output enables based on the overcurrent flags and internal state. When an output is first turned on the overcurrent flag is ignored for a programmable delay (usually a few ms) in order to avoid false triggering from inrush spikes. After this period, if the overcurrent flag is ever asserted the channel is turned off and placed in the "error-disable" state. In order to clear an error condition the channel must be manually cycled, much like a conventional circuit breaker.

Here's a view of the finished first-run prototype. As you can see the first layout revision had a few bugs ;) The dead-bugged oscillator turned out to not be necessary but it would have been more work to remove it so I'm keeping it until I do a respin with all of these fixes incorporated.
PDU board on my desk
The SNMP interface and IP protocol stack runs on a custom softcore CPU of my own design. The CPU is named GRAFTON, in keeping with my tradition of naming my processors after nearby towns. It is fairly similar to MIPS-1 at the ISA level and can be targeted by mips-linux-gnu gcc with carefully chosen flags, but does not implement unaligned load/store, interrupts, or the normal coprocessors. Coprocessor 0 exists but is used to interface with the RPC network.

GRAFTON's programming model is largely event-driven, in a model that will be somewhat familiar to anyone who has done raw Windows API programming. The CPU sleeps until an RPC interrupt packet shows up, then it is processed and it goes back to sleep. Unlike classical interrupt handling, user code running on GRAFTON cannot be pre-empted by an interrupt; it just sits in the queue until retrieved.

int main()
	//Do one-time setup

	//Main message loop
	RPCMessage_t rmsg;
	return 0;

RPCFunctionCall(), a simple C wrapper around the low-level SendRPCMessage and RecvRPCMessage() functions, abstracts the RPC network with a blocking C function call semantics. Any messages other than return values of the pending call are queued for future processing.

In the example below, I'm initializing the SPI modules for the A/D converters with a clock divisor computed on the fly from the system clock rate.

void ADCInitialize()
	//SPI clock = 250 KHz
	RPCMessage_t rmsg;
	RPCFunctionCall(g_sysinfoAddr, SYSINFO_GET_CYCFREQ, 0, 250 * 1000, 0, &rmsg);
	int spiclk =[1];
	for(unsigned int i=0; i<3; i++)
		RPCFunctionCall(g_periphAddr, PERIPH_SPI_SET_CLKDIV, spiclk, i, 0, &rmsg);

The firmware is about 4300 lines of C in total, including comments but not the 1165 lines of C and assembly in my C runtime library shared by all GRAFTON designs. It implements IPv4, UDP, DHCP, ARP, ICMP echo, and SNMPv2c. SNMPv3 security and IPv6 are planned but are on hold until I move firmware out of block RAM and into flash so I have some space to work in. Other than that, it's essentially feature-complete and I've been using the PDU in my lab for a while while working on my flash controller and some support stuff.

The PC-side UI, intended to control several PDUs, is written in C++ using gtkmm and communicates with the board over SNMP. One tab (not shown) contains summary information with one graph trace per PDU.

PDU control panel
With a few minutes of PHP scripting I was also able to get my Munin installation to connect to the PDU and collect long-term logs even when I don't have the panel up.

Munin logs of PDU
The board runs quite cool, the spikes of heat caused by my furnace kicking in are quite visible and dwarf thermal variations caused by changes in load.

It needs a little bit more work to be fully production-ready but is already saving me time around the lab.

My desk with the PDU installed
Here's a look at my desk after deploying the PDU. The power cable mess is almost completely gone :) I do need to tidy up the Ethernet cables at some point, though...

1 comment:

  1. Very impressive stuff! Look forward to hearing more about the processor design.