A basic Micro-controller

[Md Mubdiul Hasan]

Hi,

Even its 1950's project, I feel interest on it. You might feel it silly!
I am trying to get attraction those people, who knows its hardware well.

Its a serial interface project, someone who posted it online.
Take a look this schematic, I have few questions.


1. Does the  resistors R1 and R2 has connection with PIN4 of  DM7406? Did you see those are connected with regulator input?
2. Whats the reason of invert signals by buffer here?
3. Does the pull-up resistors are on matched value against IN/OUT current and voltage ? I got MAX and MIN pull-up resistor value setting equation on DM7406.
4. Anything wrong you could see in this design?

Kindly post your comment here.

[Md Mubdiul Hasan]

Dear Sir Blueskull,

Thank you for nice technical descriptions.
Let me draw my interest on your comments right here.

1.

7406 is an open collector gate, so output pull-ups are needed. In this particular design, you get selectable 0V/4.8V/12V outputs.

-Open collector chips have no internal pull-up resistor, right? Where you would get such different output range in this design?
Dont you mean A1,Y1......An,Yn?

2. 

trigger a low active interrupt

, why we need to trigger you know?

3.

It depends, but it is NOT critical. Any value in the range may work. A higher resistance makes it less reliable and slower, while a lower resistance makes it consuming more power. Just like I2C bus pull-ups, it is a speed-vs-power balance.

Cant we calculate resister values ? Can you check datasheet kindly?

4.

Didn't see local input decoupling caps on all chips, make sure you have them in final layout. Digital glitch is hard and expensive to find, so make sure they don't happen in the first place. C1/C2 need to be chosen according to specific IC and crystal, make sure you did the math.

Please identify them. Dont you mean RC calculation/time constant/frequency calculation?

but don't forget VCC and GND of chips in your layout.

Yes, well noticed.

[rstofer]

The circuit seems to be a chip programmer.  In the old days (and still with some PICs), high voltage was applied during programming.  Hence the 12V capability and the use of open collector inverters to select the voltage applied to Vpp.

I used the 80C251 about 12 years ago and it was a pretty nice chip.  I certainly wouldn't use it today!  OTOH, I have a very nice Pascal compiler for the 8051 family and I really like the language.