[MinnowBoard] 1.6V output on gpio pin

Ken Tait ktait at venturesystemdesign.com
Mon Feb 8 16:30:26 UTC 2016


your welcome!

A little tutorial if you are interested.

All current microprocessors and microcontrollers have non specific port 
drive capabilites. what I mean by this is that all data sheets are a 
little vague about what a pin can drive,(actually sink). This is due to 
the complex structure used to enable a pin to be multi-function and the 
low power current requirements.
A part is specified by how much heat it can tolerate which is directly 
related to it's speed and how much load is on ALL the pins. So, if you 
had 20 leds at 20ma on 20 pins, you would exceed the specs quickly...
NO pins are designed to SOURCE current,(only enough to drive a buffer) 
just to SINK current. This is an important aspect to a design.

As dies shrink, the pin drive capability gets worse, since it takes 
physical area on the die to sink current.
The newer ARM parts are making this worse still, since they are all 
heading to the lowest possible power use and smallest die size for cost....

8 bit micros can easily sink 10ma of current, many have a few pins 
defined as 'high drive' that will do more than that.This is mostly for 
driving leds directly in low cost consumer goods. It is easier to do 
this in this type of part, it has fewer transistors so die size is not 
that much of an issue.

The X86 family is not designed to drive anything directly without a 
buffer, that is not it's purpose. Older parts like 80186 derivatives 
could do this because they had very different output pins and were 
intended to be used in small systems. True 'cpu' level parts are 
designed to drive memory arrays and nothing else.

To be safe and provide for future expansion, any X86 design should have 
gpio pins buffered to the outside world. A dual voltage, 8 bit 
bi-directional buffer is a good choice here since you can mate the 1.8v 
pin with a 5V or 3.3V output.

If you pay attention to this, then when a newer, faster part comes out 
it will almost be a drop in replacement with little board re-layout 
required.

I see this issue all the time in systems I work on. Designers do not 
understand the newer parts and what their requirements are, thinking 
they can treat them like the older parts.

/Knowledge is power..and produces less smoke. ;-) /
Hope this helps
Ken


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