
Inside the 8086 processor, tiny charge pumps create a negative voltage - zdw
http://www.righto.com/2020/07/inside-8086-processor-tiny-charge-pumps.html
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userbinator
Not only is the "back bias" "helpful", it is _necessary_ to prevent the
transistors from conducting when they shouldn't, which could lead to a latch-
up and catastrophic failure; hence the power sequencing requirements of these
ICs.

(If you search online you'll find discussions such as
[https://electronics.stackexchange.com/questions/455745/why-w...](https://electronics.stackexchange.com/questions/455745/why-
would-a-intel-8080-chip-be-destroyed-if-12-v-is-connected-before-5-v) where
the accepted answer is close but not quite correct; -5 is substrate bias, 5 is
the main supply, and 12 is applied to all the gates of the enhancement load
transistors to open them. On the 8080, 12V was also used to drive the clocking
circuitry, and this explains why the absolute maximum rating on all the pins
--- and thus the gate oxide withstand voltage --- is 20V instead of the 6-7V
typically seen for 5V devices.)

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peter_d_sherman
>"You might wonder how a charge pump can turn a positive voltage into a
negative voltage. The trick is a "flying" capacitor, as shown below. On the
left, the capacitor is charged to 5 volts.

 _Now, disconnect the capacitor and connect the positive side to ground. The
capacitor still has its 5-volt charge, so now the low side must be at -5
volts._

By rapidly switching the capacitor between the two states, the charge pump
produces a negative voltage."

That is a strange and weird effect of capacitors!

I never knew this, before this post!

 _I 'll have to experiment with this weird but interesting effect in the
future!_

~~~
osamagirl69
Most all modern rs232 ports use a similar approach to generate the negative
voltages needed as well. The ubiquitous max232 and its copies/clones all use
charge pumps to convert a +5v rail to +/-12v rails for the line drivers.

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kens
Take a look at the max233 cross-sections by @TubeTimeUS. It looks like a
normal DIP integrated circuit, but it has four discrete capacitors in the IC
package for the charge pumps.

[https://twunroll.com/article/1286136812658806785#](https://twunroll.com/article/1286136812658806785#)

~~~
osamagirl69
Very cool -- I always wondered how they integrated those caps into the package

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egsmi
Kens, I love the series on the 8086. For the next article can you go over how
Intel did clock distribution?

~~~
kens
I was actually thinking about a short blog post on the 8086's clock circuit.
The short answer is that the clock signal into the 8086 goes through a bunch
of inverters to strengthen and shape the signal, and then two-phase clock
signals (i.e. clock and clock') wind around the chip.

Many processors of that era used pass transistors for temporary latches
between circuits. The 8086 also used dynamic logic for gates, where instead of
a pull-up resistor, the output would be precharged during one clock phase and
then the gate would pull it down (or not) during the other clock phase.

~~~
egsmi
How interesting. I didn’t think differential clocks where a thing back then.

Or maybe they’re not differential and they’re just routed together because
it’s convenient. Since they store with coupled inverters I can see why having
both phases could be useful in some flip flop implementations.

~~~
userbinator
[https://en.wikipedia.org/wiki/Gated_latch#Master–slave_edge-...](https://en.wikipedia.org/wiki/Gated_latch#Master–slave_edge-
triggered_D_flip-flop)

The two clock phases don't overlap, and act to "pump" the signals through the
circuits on each half-cycle. Circuits from that era effectively use both edges
of the clock, and the use of simple pass transistors instead of full flip-
flops simplifies the implementation.

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aidenn0
Charge pumps are still found in many places, including the high-side of an
h-bridge (or half h-bridge) driver, as that allows for using only NMOS for
switching.

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smitty1e
> 3\. Prototype designs always work.

Looked jolly awesome on the whiteboard, though.

~~~
formerly_proven
I assume this isn't a joke, but rather refers to handfitted prototypes
working, where a massproduced item would not without significant engineering:
if you tweak it until it works, it works.

~~~
smitty1e
I interpreted it to mean that the on-paper design can hang together nicely,
until it's instantiated in some hardware, followed by the "Oh, yeah. . ."
moment.

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raphlinus
Again, the number filter mangled the title. The full title from the article is
"Inside the 8086 processor, tiny charge pumps create a negative voltage". In
the interest of brevity, "processor" could be omitted or replaced with "chip."

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nickff
“Chip” is actually a term of art in the industry, used to describe ‘single
passives’ (such as resistors and capacitors) in small SMD packages. There are
machines called “chip shooters” which are designed to place those ‘chips’.

Using the term “chip” to describe integrated circuits is amateurish.

~~~
Treblemaker
Perhaps so in the context of PCB assembly. In the semiconductor industry we
still call them chips. So do Micron Semiconductor [1], TSMC [2] and Intel [3]

[1]
[https://www.micron.com/foundation/semiconductors](https://www.micron.com/foundation/semiconductors)

[2]
[https://www.tsmc.com/english/dedicatedFoundry/technology/SoI...](https://www.tsmc.com/english/dedicatedFoundry/technology/SoIC.htm)

[3] (pdf)
[http://download.intel.com/pressroom/kits/chipmaking/Making_o...](http://download.intel.com/pressroom/kits/chipmaking/Making_of_a_Chip.pdf)

~~~
pwg
And the usage goes back to at least August 1975 (likely earlier):

[http://archive.6502.org/datasheets/mos_6501-6505_mpu_prelimi...](http://archive.6502.org/datasheets/mos_6501-6505_mpu_preliminary_aug_1975.pdf)

"but including an _on-chip_ clock ... in addition to the _on-chip_ clock"

~~~
kps
Two unrelated papers in _AFIPS '65_ use ‘chip’ in their abstracts, in the
integrated circuit sense, and neither draws attention to the term.

[https://dl.acm.org/doi/10.1145/1463891.1464005](https://dl.acm.org/doi/10.1145/1463891.1464005)
[https://dl.acm.org/doi/10.1145/1463891.1463961](https://dl.acm.org/doi/10.1145/1463891.1463961)

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binbag
Nice article

