Why are so many pins of 8086 active-low? Does it have to do something with the fact that more circuits are able to sink current than to source current, or is there another reason for it?
Electronic – Why are so many pins of 8086 active-low
digital-logiclogic-levelmicroprocessor
Related Solutions
Look at what happens during power-up: Vcc rises to a point where it's high enough to make everything work properly. However, that point isn't clearly defined and may vary from device to device. It makes sense not to use this voltage to reset the controller.
It's easy, however, to keep a level low regardless of Vcc. After all, Reset is already active the instant you switch power on, since at that moment everything is at a low level.
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The graph below illustrates how the output voltage of the reset controller (i.c. an MC34064) remains low until Vcc is high enough to have the complete microcontroller stable.
You can get the 8088 from futurlec.com for about $4. Its an 8086 with an 8-bit external bus.
Try searching for "NEC v20" which was an 8086 clone that was popular.
The 80186 is the embedded system-on-chip equivalent, there's tons of variants of that still in use.
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Best Answer
In the TTL era, active low was the standard.
The old NMOS parts like the 8086 were designed in that time period- note that the input levels are TTL-compatible Vih=2.0V and Vil=0.8V.
Active-low allows wired-OR with open collector/drain outputs (you may have noticed that calling it 'wired-OR' implies negative logic).
Also, the NMOS parts were almost like RTL except the pull-up resistors (loads) were depletion mode n-channel MOSFETs so the negative edges would tend to be sharper with capacitive load. There are no p-channel parts in this technology (which made it simpler and thus higher yield)
CMOS parts (originally 4000 series) were not designed to be TTL compatible and generally have active-high inputs. The 74C/74HC etc CMOS parts derived from similar TTL functions tend to have active-low inputs. Remembering this difference can serve you even today- if you need a flip-flip with active high clear input you are more likely to find it in a 4xxx or 74HC4xxx part than in a 74HCxx part such as 74HC74.