6821 PIA

[P Colyer]

Is the input impedance of CB2 in input mode true Hiz meaning an external
pull up is required like the PB0 - PB7 lines;
or is it functionally the same as a TTL input ?

The data sheet I have is a little ambiguous.

[Bill Janssen]

Post by P Colyer
Is the input impedance of CB2 in input mode true Hiz meaning an external
pull up is required like the PB0 - PB7 lines;
or is it functionally the same as a TTL input ?
The data sheet I have is a little ambiguous.

I don't understand your concern. If the connected device can drive a
high Z load then no
pull up is needed. It depends on the driving circuit.

Bill K7NOM

[P Colyer]

I shoulda rephrase it differently

is the CB2 a open collector output? - this is the only I/o avaialable to me
to use a Dallas 1W device.
1W devices require a pull up of about 4K7 and with the same line I want to
be able to pull it to 0V when CB2
is in the output mode.

Post by Bill Janssen

Post by P Colyer
Is the input impedance of CB2 in input mode true Hiz meaning an external
pull up is required like the PB0 - PB7 lines;
or is it functionally the same as a TTL input ?
The data sheet I have is a little ambiguous.

I don't understand your concern. If the connected device can drive a
high Z load then no
pull up is needed. It depends on the driving circuit.
Bill K7NOM

[Bill Janssen]

Post by P Colyer
I shoulda rephrase it differently
is the CB2 a open collector output? - this is the only I/o avaialable to me
to use a Dallas 1W device.
1W devices require a pull up of about 4K7 and with the same line I want to
be able to pull it to 0V when CB2
is in the output mode.

Oh, OK that makes more sense. I don't have my book here (it's someplace
I don't remember)
but I used the CB2 to drive a old version of TTL chip with out using a
pull up resister. As I remember
some of the leads had limited drive capability but they could all drive
at least one TTL load.

I hope that helps.

Bill K7NOM

[ErnieP]

Post by Bill Janssen

Post by P Colyer
I shoulda rephrase it differently
is the CB2 a open collector output? - this is the only I/o avaialable to me
to use a Dallas 1W device.
1W devices require a pull up of about 4K7 and with the same line I want to
be able to pull it to 0V when CB2
is in the output mode.

Oh, OK that makes more sense. I don't have my book here (it's someplace
I don't remember)
but I used the CB2 to drive a old version of TTL chip with out using a
pull up resister. As I remember
some of the leads had limited drive capability but they could all drive
at least one TTL load.
I hope that helps.
Bill K7NOM

I've gat a genuine Motorola MEK6802D5 evaluation baord here with, of all
things,
a genuine Hitachci HD46821P on it. The debug ROM on the board allows me to
poke control register values.

Using an oscilloscope to observe and with CB2 programmed as an input, CB2 is
statically low but can easily be driven high with just the 60 hz from me
touching the
pin - definitely hiZ. As a side note, the pin appears to be Open Drain in
that I can
easily drive it above the 5 volt supply.

When I program it as low output, it goes low and stays there with no ready
evidence
of my finger touching it. When the pin is set as an output high it does go
to approx
VCC but I can still drive it higher with my finger touch.

I looked at the CA2 pin while I was at it and it appears to have an internal
pullup as
stated by the Motorla MC6821 data sheet I have.

[P Colyer]

Okay thanks folks to all that replied-

I hope that the Hitachi HD46821 has the same specs as the motorola MC6821 on
the board I have.
Most spec sheets I have found, tell when CB2 is an input "states that it has
a high impedance input and is compatible with standard TTL"
meaning it acts like a TTL input - thats confusing.

Post by ErnieP

Post by Bill Janssen

Post by P Colyer
I shoulda rephrase it differently
is the CB2 a open collector output? - this is the only I/o avaialable

to

Post by ErnieP
me

Post by Bill Janssen

Post by P Colyer
to use a Dallas 1W device.
1W devices require a pull up of about 4K7 and with the same line I want

to

Post by Bill Janssen

Post by P Colyer
be able to pull it to 0V when CB2
is in the output mode.

Oh, OK that makes more sense. I don't have my book here (it's someplace
I don't remember)
but I used the CB2 to drive a old version of TTL chip with out using a
pull up resister. As I remember
some of the leads had limited drive capability but they could all drive
at least one TTL load.
I hope that helps.
Bill K7NOM

I've gat a genuine Motorola MEK6802D5 evaluation baord here with, of all
things,
a genuine Hitachci HD46821P on it. The debug ROM on the board allows me to
poke control register values.
Using an oscilloscope to observe and with CB2 programmed as an input, CB2 is
statically low but can easily be driven high with just the 60 hz from me
touching the
pin - definitely hiZ. As a side note, the pin appears to be Open Drain in
that I can
easily drive it above the 5 volt supply.
When I program it as low output, it goes low and stays there with no ready
evidence
of my finger touching it. When the pin is set as an output high it does go
to approx
VCC but I can still drive it higher with my finger touch.
I looked at the CA2 pin while I was at it and it appears to have an internal
pullup as
stated by the Motorla MC6821 data sheet I have.

[james]

+++Okay thanks folks to all that replied-
+++
+++I hope that the Hitachi HD46821 has the same specs as the motorola MC6821 on
+++the board I have.
+++Most spec sheets I have found, tell when CB2 is an input "states that it has
+++a high impedance input and is compatible with standard TTL"
+++meaning it acts like a TTL input - thats confusing.
+++
+++

****************]

Motorola licensed Hitachi to second source a lot of their ICs during
the 80's. Also ST Micro and a few others second sourced the
MC6809/MC6821/ and several other of that line. All licensed parts meet
Motorola specs. There could be minor differences that would not affect
most operations.

Hitachi HD68xxxx parts are High density NMOS Depletion load ICs. The
HD63xxx series of parts are CMOS parts. I also believe that the HD468
series are also NMOS.

james

+++
+++
+++>
+++> >
+++> > >I shoulda rephrase it differently
+++> > >
+++> > >is the CB2 a open collector output? - this is the only I/o avaialable
+++to
+++> me
+++> > >to use a Dallas 1W device.
+++> > >1W devices require a pull up of about 4K7 and with the same line I want
+++> to
+++> > >be able to pull it to 0V when CB2
+++> > >is in the output mode.
+++> > >
+++> > >
+++> > >
+++> > Oh, OK that makes more sense. I don't have my book here (it's someplace
+++> > I don't remember)
+++> > but I used the CB2 to drive a old version of TTL chip with out using a
+++> > pull up resister. As I remember
+++> > some of the leads had limited drive capability but they could all drive
+++> > at least one TTL load.
+++> >
+++> > I hope that helps.
+++> >
+++> > Bill K7NOM
+++>
+++> I've gat a genuine Motorola MEK6802D5 evaluation baord here with, of all
+++> things,
+++> a genuine Hitachci HD46821P on it. The debug ROM on the board allows me
+++to
+++> poke control register values.
+++>
+++> Using an oscilloscope to observe and with CB2 programmed as an input, CB2
+++is
+++> statically low but can easily be driven high with just the 60 hz from me
+++> touching the
+++> pin - definitely hiZ. As a side note, the pin appears to be Open Drain in
+++> that I can
+++> easily drive it above the 5 volt supply.
+++>
+++> When I program it as low output, it goes low and stays there with no ready
+++> evidence
+++> of my finger touching it. When the pin is set as an output high it does
+++go
+++> to approx
+++> VCC but I can still drive it higher with my finger touch.
+++>
+++> I looked at the CA2 pin while I was at it and it appears to have an
+++internal
+++> pullup as
+++> stated by the Motorla MC6821 data sheet I have.
+++>
+++>
+++>
+++>
+++

[james]

On Sun, 9 Jul 2006 17:17:54 -0400, "ErnieP"

+++
+++>
+++> >I shoulda rephrase it differently
+++> >
+++> >is the CB2 a open collector output? - this is the only I/o avaialable to
+++me
+++> >to use a Dallas 1W device.
+++> >1W devices require a pull up of about 4K7 and with the same line I want
+++to
+++> >be able to pull it to 0V when CB2
+++> >is in the output mode.
+++> >
+++> >
+++> >
+++> Oh, OK that makes more sense. I don't have my book here (it's someplace
+++> I don't remember)
+++> but I used the CB2 to drive a old version of TTL chip with out using a
+++> pull up resister. As I remember
+++> some of the leads had limited drive capability but they could all drive
+++> at least one TTL load.
+++>
+++> I hope that helps.
+++>
+++> Bill K7NOM
+++
+++I've gat a genuine Motorola MEK6802D5 evaluation baord here with, of all
+++things,
+++a genuine Hitachci HD46821P on it. The debug ROM on the board allows me to
+++poke control register values.
+++
+++Using an oscilloscope to observe and with CB2 programmed as an input, CB2 is
+++statically low but can easily be driven high with just the 60 hz from me
+++touching the
+++pin - definitely hiZ. As a side note, the pin appears to be Open Drain in
+++that I can
+++easily drive it above the 5 volt supply.
+++
+++When I program it as low output, it goes low and stays there with no ready
+++evidence
+++of my finger touching it. When the pin is set as an output high it does go
+++to approx
+++VCC but I can still drive it higher with my finger touch.
+++
+++I looked at the CA2 pin while I was at it and it appears to have an internal
+++pullup as
+++stated by the Motorla MC6821 data sheet I have.
+++
+++
+++

^^^^^^^^^^^^^^^^^

First off the 6821 uses NMOS Depletion Load technology. It is not
CMOS, but instead high density NMOS IC. Niether the CA1 or the CB1 are
open collector. Instead they are configured as inputs only. CA2 and
CB2 when configured as an output are not open drain. Instead the
output circuit most likely uses NMOS depletion load. These have only
one TTL load capacity. When configured as an input the fan in is 1.5
TTL. Yes there would be a weak pullup internal. I would suspect that
most of the resistance is in any ESD protection circuitry on the die
near the bonding pad. It could be as simple as a biased zener diode.

The MC6821 is an improvement and second generation of the MC6820. The
MC6822 did have open drain on its outputs and would handle pullup
voltages of up to 15 VDC if I remember correctly.

hope this helps
james