Thursday, November 7, 2013

Brain Cortex Propeller Protection Part 26

PROTECT THE PROP Can you use a raw Propeller chip right out of the box? No, not if you're going for immortality and cannot afford to have it die along the way! You'll need to properly cook, giving the proper condiments to ensure it provides the immortal performance required in the brain cortex.

If you follow along at the Parallax Propeller Forum, you will discover a number of very important points, mainly component and circuit additions to add to the chip and some rules to follow. Let's take a look at some of the most important points to increasing prop longevity and offer up some useful technical quotes by Mike Green from the Forum.

"There's no maximum times the Prop can boot up and no maximum times the EEPROM can be read. Microcontrollers are very very sensitive to voltages being connected to I/O pins without power being applied to the power pins. This is true for any digital chips and most analog chips not specifically designed for this. The protective diodes on the I/O pins are rated for only 50uA and voltages in excess of 4V (for a 3.3V part like the Propeller) can be instantly fatal."

"If the 3.3V sensors are powered and the Propeller is not, you can still get into trouble because Vdd for the Propeller is 0V and the input voltage is 3.3V. That will burn out the protective diodes and may damage other Prop-internal circuitry as well. 5V sensors can also burn out the protective diodes since about 1mA can flow through the 4.7K resistors when Vdd is 0V and the diodes are only rated for 500uA. The 4.7K resistors limit the current to 500uA only when the Prop's Vdd is greater than 2.5V."

"Basically, these measures protect the Prop adequately only when the Prop is powered. You need more protection if the Prop can possibly be unpowered when its inputs are powered..."

"I strongly suggest having a 0.1uF ceramic capacitor connected between each pair of Vdd/Vss pins on either side of the package. The fact that your setup works doesn't mean that it will work reliably. The Propeller is made up of circuitry that switches very rapidly and even a few inches of wire between the package power supply pins and the power source can introduce enough inductance plus the battery's internal resistance so the supply voltage can drop very briefly very significantly. The 0.1uF capacitors mounted as close as possible to the package pins act as a high speed power supply buffer for these transient power demands." 

"The Propeller was felt to be a bit more robust in the past. The "PLL multiplexor" failure mode demonstrated the importance of connecting both sets of power supply pins using short PCB traces and bypass capacitors, that the absence of these measures had more consequences than just more noise."

"There was some discussion of this when it was first discovered. Chip looked at the design and decided that voltage drops across the chip in the power distribution network were responsible when either one set of Vdd/Vss pins were not connected or when the bypass capacitors were missing or too far from the chip. The PLL multiplexer was the most vulnerable just because of where it lay in the power distribution network. If the chip was rearranged, some other structure would be the primary failure point. What happens is that a portion of the chip transiently is powered by Vdd/Vss less a voltage drop across the chip while some of the inputs of that portion come from another section of the chip without the voltage drop and that input voltage exceeds the limits of the transistor structures involved since they're powered via the voltage drop and they eventually fail. When the two pairs of Vdd/Vss pins are connected together and to the power source via a low impedance connection (as the chip is designed for), the voltage drop across the chip is reduced to where this problem doesn't occur."

Here's the checklist generated at Humanoido Labs to add Propeller chip protection and increase longevity.

* two decoupling capacitors .1uf
* crisscross chip wiring VSS to VSS and VDD to VDD
* filtering capacitors 1000uf, 3300 uf,  1 uf, .1uf
* Prop is never powered down with pins powered
* minimized leads
* protection resistors
* blocking diode as needed
* transient control
* control of over voltage and under voltage
* matching power demands
* not exceeding current pin chip specs

Index to the Brain Cortex