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Help Ram connections !!!!

B

BSL

Jan 1, 1970
0
Hi Guys , i have to use a 8 bits SRam to fix a 4 bits design , on the
new ram i will have 4 data lines not use ,so Is it safe to leave
these pins floating ??? can the Ram be unstable if they're not pulled
up or down ???

Thanks
 
E

Eeyore

Jan 1, 1970
0
BSL said:
Hi Guys , i have to use a 8 bits SRam to fix a 4 bits design , on the
new ram i will have 4 data lines not use ,so Is it safe to leave
these pins floating ??? can the Ram be unstable if they're not pulled
up or down ???

Pull them one way or the other (it doesn't matter which).

Inputs left open/floating may cause various problems.

Graham
 
B

BSL

Jan 1, 1970
0
Pull them one way or the other (it doesn't matter which).

Inputs left open/floating may cause various problems.

Graham

Would it be a good idea to tie the not use ones with adjacent use
ones ??? let's say d1 tie to d2 !!!
 
E

Eeyore

Jan 1, 1970
0
BSL said:
Would it be a good idea to tie the not use ones with adjacent use
ones ??? let's say d1 tie to d2 !!!

You'll double the input capacitance that way which might be problematic.

Graham
 
B

BSL

Jan 1, 1970
0
You'll double the input capacitance that way which might be problematic.

Graham

The problem i have is that i must do a small adapter boards and don't
want a put many resistors on it !!!
 
J

John Fields

Jan 1, 1970
0
Hi Guys , i have to use a 8 bits SRam to fix a 4 bits design , on the
new ram i will have 4 data lines not use ,so Is it safe to leave
these pins floating ??? can the Ram be unstable if they're not pulled
up or down ???
 
E

Eeyore

Jan 1, 1970
0
BSL said:
The problem i have is that i must do a small adapter boards and don't
want a put many resistors on it !!!

If you're not driving it very fast, then that may be fine.

Why the problem with resistors though ? Couldn't they be small surface mount
types ?

Graham
 
B

BSL

Jan 1, 1970
0
If you're not driving it very fast, then that may be fine.

Why the problem with resistors though ? Couldn't they be small surface mount
types ?

Graham- Hide quoted text -

- Show quoted text -

:) yes smt are small !!, just wanted to put less parts as possible ,
btw the max speed of the cpu is merely 1 MHz , it is an old slow
system , the ram is a lo power used to keep status while power off , a
memory cap is the power keeper !!!!
 
:) yes smt are small !!, just wanted to put less parts as possible ,
btw the max speed of the cpu is merely 1 MHz , it is an old slow
system , the ram is a lo power used to keep status while power off , a
memory cap is the power keeper !!!!

Just ground the unused bits. No resistors needed.
 
K

krw

Jan 1, 1970
0
---
I'd pull them either up or down (I prefer down) through largish
(10k?) resistors since you probably won't know whether the part will
be READ or WRITE during power-up and you won't want to short the
pins to Vcc or GND if it comes up WRITE.

Why does it matter? Are the pins Bi-Di (then read would be a
problem)? CMOS inputs shouldn't matter where they're tied to as long
as it's inside the rails and not in the threshold region. I wouldn't
short inputs together because it would increase fan-in and power
consumption (slightly) for no advantage.
 
B

BSL

Jan 1, 1970
0
Why does it matter? Are the pins Bi-Di (then read would be a
problem)? CMOS inputs shouldn't matter where they're tied to as long
as it's inside the rails and not in the threshold region. I wouldn't
short inputs together because it would increase fan-in and power
consumption (slightly) for no advantage.

So Keith , do i leave the 4 unused data bits floating ???
 
R

Rich Grise

Jan 1, 1970
0
So Keith , do i leave the 4 unused data bits floating ???

No, you never ever leave any chip input floating. In this case, since
it's RAM, strapping the pins to either Vee or Vcc would be OK, as long
as you perform a write to each byte before trying to read it - that way,
you won't blow out a pin trying to drive it against a short. You might
say that these pins are "don't care", but who knows what happens to the
rest of the chip when one of the output drivers gets blown?

But in most cases, I recommend pullup resistors - in this case, probably
something in the range of 22K ~ 100K, since it's CMOS and doesn't draw
any significant current.

Hope This Helps!
Rich
 
K

krw

Jan 1, 1970
0
So Keith , do i leave the 4 unused data bits floating ???

Never! A floating pin often wants to go towards the middle and it
may draw lots of current. Tie them one way or the other.
 
J

John Fields

Jan 1, 1970
0
Why does it matter?

---
Because if the chip powers up in WRITE and you don't know what'll be
in the cell array, You could short the I/O pin to either Vcc or GND
with the I/O pin in the opposite state.

Also read Rich Grise's post for more detail.
---
Are the pins Bi-Di (then read would be a
problem)?

---
No, WRITE would. See above.
---
CMOS inputs shouldn't matter where they're tied to as long
as it's inside the rails and not in the threshold region.
 
J

John Fields

Jan 1, 1970
0
It won't matter when the RAM's I/O are inputs but, remember, it's a
RAM so its I/O will be outputs when it's in WRITE mode.
---
 
K

krw

Jan 1, 1970
0
Now I see, assuming Bidirectional I/O. I got the impression that the
RAM in question had separate I/Os; ground the gazintas and float the
gazoutas.
 
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