Preventing back powering through ICs

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Speculatrix

Joined Mar 30, 2018
11
First, I'm a real beginner, so please be gentle.

I'm working on a board that will use a microcontroller (ATMEGA328PB) to act as a 'smart' interface allowing a Raspberry Pi to print to my trusty old Epson MX-80 dot matrix printer. It uses a 74HC595 shift register to convert serial input to the eight parallel data lines. And other signals to/from the printer go via a 74HC541 buffer IC.

And my prototype works, but with one problem. If the printer is powered up, but the interface board isn't, the shift register and buffer chips are still being powered and also feeding power to the main VCC rail.

The reason for this this is that the Epson holds a lot of the signal lines high by default (I presume there are pullups in the printer). I'm assuming that these are driving the VCC pins of the ICs high via their built-in ESD diodes. But I'm at a loss as to what to do about that.

We're dealing with a mix of signals - some inputs to the printer, some outputs from it. Most are active low, which is why they're being pulled high by the printer by default. So, try as I might, I can't work out how simple diodes would help here.

My googling on the subject has led me down many complex paths, mostly involving mosfets, but all 'solutions' I've found so far have been above my pay grade in terms of complexity. Am I overlooking something simple?

I'm at the point where I'm tempted to just slap a label on the board saying 'power board before turning on printer'. (This is a personal project, not a commercial one.)

I've made a number of posts about this project here: https://mansfield-devine.com/speculatrix/category/projects/smartparallel/

And there's a Github page with the KiCad files here: https://github.com/mspeculatrix/SmartParallel

And I've attached a schematic.

Any pointers to useful reading/tutorials would be appreciated. I don't need a canned solution - just some hints to help me understand how to solve this.
 

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kubeek

Joined Sep 20, 2005
5,733
There are logic families that allow for partial power down and should not back-power your circuit. For example SN74LVC1G34 but there will be others more suitable, just check the TI website.
 

Joël Huser

Joined Jun 30, 2019
42
First, I'm a real beginner, so please be gentle.

I'm working on a board that will use a microcontroller (ATMEGA328PB) to act as a 'smart' interface allowing a Raspberry Pi to print to my trusty old Epson MX-80 dot matrix printer. It uses a 74HC595 shift register to convert serial input to the eight parallel data lines. And other signals to/from the printer go via a 74HC541 buffer IC.

And my prototype works, but with one problem. If the printer is powered up, but the interface board isn't, the shift register and buffer chips are still being powered and also feeding power to the main VCC rail.

The reason for this this is that the Epson holds a lot of the signal lines high by default (I presume there are pullups in the printer). I'm assuming that these are driving the VCC pins of the ICs high via their built-in ESD diodes. But I'm at a loss as to what to do about that.

We're dealing with a mix of signals - some inputs to the printer, some outputs from it. Most are active low, which is why they're being pulled high by the printer by default. So, try as I might, I can't work out how simple diodes would help here.

My googling on the subject has led me down many complex paths, mostly involving mosfets, but all 'solutions' I've found so far have been above my pay grade in terms of complexity. Am I overlooking something simple?

I'm at the point where I'm tempted to just slap a label on the board saying 'power board before turning on printer'. (This is a personal project, not a commercial one.)

I've made a number of posts about this project here: https://mansfield-devine.com/speculatrix/category/projects/smartparallel/

And there's a Github page with the KiCad files here: https://github.com/mspeculatrix/SmartParallel

And I've attached a schematic.

Any pointers to useful reading/tutorials would be appreciated. I don't need a canned solution - just some hints to help me understand how to solve this.
Wow Speculatrix ! I like your way to draw your schematic ! I am very similar to you.....

Kicad.....
 

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