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Posts: 9
Registered: ‎07-29-2016
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Power options error in EM35x Dev Kit Breakout Board

Doing some work using EM35x breakout board, I think I've stumbled across a design error.  I'm looking at a rev C2 board, and rev C2 of the schematic.

 

The problem is the way power is routed from the ISA connector.

 

First, a minor problem with the documentation, TS6.  In section 2.1, it says the debug adapter can power the target through J31, the packet trace port.  In fact, it seems to be able to power the board through either J31 or J28 (the DEI port).  On the schematic, J31.1 and J28.1 both connect to the VDD_3V_BB net.

 

And that connection seems to be wrong, to me.  I think J31.1 and J28.1 should be connected to VDD_ISA, not VDD_3V_BB.  VDD_3V_BB is the output of the module power selector jumper (J2, J3).  The inputs to the jumper are VIN_BATT, VIN_REG, and (supposedly) VIN_ISA.

 

Because the ISA power is connected to the wrong side of the power-select jumper, it will always power the board, even if you have set the select jumper to VIN_BATT or VIN_REG, or remove the select jumper entirely.  And you'll have two different supplies shorted together, which is exactly what the power-select jumper is supposed to prevent.

 

To demonstrate this, just set the power-select jumper to VIN_BATT, but don't connect a battery.   Connect to the ISA debug adapter.  The board should have no power, but you'll find it does.  And the VIN_BATT LED will be lit up too (this happens because power is basically running backwards through the power-select jumpers, from VDD_3V_BB to VIN_BATT.)

 

I guess this probably can be avoided if you're sure to set the ISA adapter's target-power switch to "Ext".  That's assuming that "Ext" disconnects power from the packet-trace and DEI ports entirely, and doesn't do something weird like short them to ground.

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Posts: 9
Registered: ‎07-29-2016

Re: Power options error in EM35x Dev Kit Breakout Board

Ok... just read a bit more in TS7, the ISA3 documentation.  Maybe the way power is routed is not actually an error.  It's necessary to make the ISA3 work.

 

It looks like when the ISA3 target power is set to "Ext", it then needs to receive the module's actual supply voltage from the target on that same pin, so it can match its voltages to the target's (because it uses 3.3V internally, regardless of the target's voltage.)  The same pin on the packet-trace and DEI ports is being used for two different purposes, depending on the setting of the Target Power switch.

 

Too bad the packet trace port doesn't have two separate pins: one to supply VDD_ISA to the breakout board, and the other to receive VDD_3V_BB back from the target.

 

I guess this is actually just a documentation problem, then.  You just have to be really, really careful to make sure the ISA3's target-power switch is set to Ext if you're using a different power supply.  Just configuring the breakout's power-select jumper is not enough by itself to prevent supply contention.