Address selection of whole cards

[Pax Writer]

Hello experts

I suspect my question is quite trivial, but here goes:
I am currently designing a uCPU-board with the possibility to add other boards onto it as piggybacks later on. Its my plan to use a common 74hc138 to select each board, but I have only had experience in selecting/deselecting single chips at a time. So here's my question: How would you suggest that I design a selection circuit for each board? Is a simple transistor inserted serially in the ground-connection enough? Maybe a MOSFET? And in that case how (since I also have no practical experience with MOSFETs, only bipolars and FETs). Maybe you have a third approach.
Please go ahead and provide suggestions. Links and schematics for examples are most welcome, and I'll do my best to answer any questions.
Thanks

 

[kchriste]

It really depends on what kind of through put and expandability you expect to achieve. Maybe look at the way the old computer ISA or PCI standard is setup.

 

[ericgibbs]

hi pax,

Is it the power lines to each piggy back pcb you are trying to select or the signal/data lines.?

If its the power lines, just switching the ground is not a good idea.

You can buy 'high side' switching devices, that will ON/OFF the +Vsupply to the piggy back pcb.
But that will leave the signal/data lines connected between the 'master' and 'slave' pcb, which can cause problems.

Post a simple drawing of a typical 'master' / 'slave' pcb interconnection.
The number of 'lines' involved will effect the answer.

EricG

 

[Pax Writer]

Rough illustration of my project

Hi again
I have attached a rough illustration of the project as a gif-image.
The bus I have drawn is common in the sense that, for example, all ICs to get access to the common 8-bit bus have connection when the piggyback-board(s) is/are mounted. Exactly what electronics will be on the piggybacks, I don't know yet, but I was hoping that each of the piggybacks will incorporate a circuit which can enable/disable (select/deselct) all electronics regardless of type with one signal line.
Am I making any sense?
Max voltage on the common bus will be standard RS232 for the serial line, +5V for digital and +12V for power.
Across all signal lines, no more than around 10mA per pin-to-pin connection is expected. For the power-nets, probably no more than 3-500mA.
No really high frequency signals as the processor will be an old atmel running at 11MHz.
I hope this clarifies things

 

[ericgibbs]

hi pax.

The best choice I can suggest for CMOS/TTL levels is the HEF40097B. Its a Tri state Non inverting octal buffer with enable lines.

Ref analog selection is the HEF4066B Quad bilateral switch.

A 74LS138 as you probably know is a momentary 'select' , you would need to latch this select line on the 'master' pcb,
toggle a FF.

If you require more select lines a HEF4515B will give you 16 selects.

The high side power switch is VN series, Tab device, a TTL logic level enable signal can switch 24V at 1 amp.

Is this along the lines that you are thinking?

Like to help.

Eric

 

[j.p.bill]

Whats' common for boards in computers is to assign addresses to each board. That way they can all share the data and address bus. Each board needs to be able to respond to data sent to some range of I/O addresses. A decoder that signals when the address range is corect is enough.

Alternatively, use unique signals sent on discreet pins to enable the boards.

You can do anything from taking buffers out of high impedance states, to turning on regulators to power up the board from complete shutdown.

 

[Pax Writer]

Hey Eric and J.P.
That was a lot of interesting information. I'll just take a moment to study the data sheets.
However, I have one general question: When an IC is disabled and in tri-state mode on the inports, does it mean that it doesn't put any load on the bus at all, or that it only puts a small load on it?
The reason I'm asking is that in this design, eventually quite many tri-statet inputs may be connected to the bus, which I fear may be too much for the outputs on the CPU to handle.

And thanks for your helpfulness so far

 

[ericgibbs]

hi pax,

The two devices I have suggested are LOCMOS, which should put a minimal load on your MCU outputs.

If you find the total 'bus' load is too high for your MCU, you can install bus drivers on the MCU pcb.

At this time I cannot find the part number for the 'high side' rail switcher device, its a 5pin tab device.

Perhaps someone else might recall the PtNo:

Eric
EDIT:
The LTC1981 high side switch, can be purchased in single or double types..

A MIC2514 would suit your application.

 

[j.p.bill]

Hi Pax,

IC's in the high impedance state won't load your bus at all. Something like an LS245 bus buffer is about the same as an open circuit.

Another dodge is to use open-collector outputs and have the bus lines individually pulled up with resistors. This works, but will limit the speed of transmissions, as the IC's have a larger load to swing. It takes significantly more space to accomodate a resistor pack, if that matters.