Cmos driving TTL through a diode

[jerryd]

Electro Tech forum,

I'm having a problem driving the clock pin of a 74LS393N
with the output pin of a TC4082BP cmos quad and gate.
The output of the gate is just a low to high to low pulse
once a minute.

It works ok if I jumper it direct but if I replace the
jumper with a diode the 74393 doesn't clock. The diode
is a 1N4148 and I'm pretty sure it's the right way around.

I'm only driving 2 other ttl pins with this gate so
fanout shouldn't be a problem. The diode doesn't
go to these other pins.

Won't have a scope until late next week.

Any suggestions?
jerryd

 

[colin55]

Why are you including a diode?

 

[MikeMl]

Why even use a diode?

(If you do use a diode, it should point at the CMOS. However, if the CMOS and TTL are both powered on 5V, then no diode is necessary.

 

[MrAl]

Electro Tech forum,

I'm having a problem driving the clock pin of a 74LS393N
with the output pin of a TC4082BP cmos quad and gate.
The output of the gate is just a low to high to low pulse
once a minute.

It works ok if I jumper it direct but if I replace the
jumper with a diode the 74393 doesn't clock. The diode
is a 1N4148 and I'm pretty sure it's the right way around.

I'm only driving 2 other ttl pins with this gate so
fanout shouldn't be a problem. The diode doesn't
go to these other pins.

Won't have a scope until late next week.

Any suggestions?
jerryd

Hello Jerry,

Well, if you replace the jumper with a Christmas tree branch it wont clock either

My question too is what is the diode for, but if you must have it then you will also need either a pull up or pull down resistor.

 

[Roff]

As others have said, why are you using a diode?
If you have the diode connected so that you are driving the anode, it will be unable to sink the TTL input current required to pull the input low. You could add a pulldown resistor at the TTL input, but it would have to be so low that the CMOS might not be able to source enough current to yield a decent logic "1" when the pulse arrives.

Post a schematic showing why you need the diode.

 

[jerryd]

Electro Tech forum,

Thanks for the inquiries.

The diode is placed between a time set switch and the
previous counter's reset pin to avoid resetting the
previous counter.

There is also a 10k resistor between the switch and ground
to prevent the insertion of electrical noise into the counter.

I've attached a schematic, Hope there's enough detail.

Jerryd

 

[Roff]

Are your counters really TTL, and not CMOS? If so, 10k will not give you a valid logic 0 at the clock input. Standard TTL logic zero input current is 3.2mA @ 0.8V. This would require a 250Ω resistor to GND. Your 4082 could not drive this.
LSTTL input current is 1.6mA @ 0.8V. This would require a 500Ω pulldown. Same problem with the 4082.

 

[audioguru]

No, Ron.
An old 74xxxx input draws a max current of 1.6mA when low. An old 74LSxxxx input draws a max current of 0.4mA when low.

The minimum output low current from a CD4xxxB ordinary logic IC is 0.51mA when its supply is 5V so most will not drive a 74xxxx input but all will drive a 74LSxxxx input.

 

[jerryd]

audioguru,

You're right, the data sheet for the 4082 has the IOL
at 0.5 ma at about 25C.

The data sheet for the SN74LS393N says the IIL for the
clock pin is -1.6 ma max @ 0.4 volts, but no minimum
or temperature is stated.

My power supply is at 5.04 volts for both VCC and VDD.

Any theories why it doesn't work with the diode?

Thanks,
Jerryd

 

[Roff]

No, Ron.
An old 74xxxx input draws a max current of 1.6mA when low. An old 74LSxxxx input draws a max current of 0.4mA when low.

The minimum output low current from a CD4xxxB ordinary logic IC is 0.51mA when its supply is 5V so most will not drive a 74xxxx input but all will drive a 74LSxxxx input.

You are quoting logic gate input currents. Look at the '393 datasheet for clock input currents.

What I said about his 4082 not being able to drive LSTTL (or standard TTL) was in relation to having to drive a logic 1 into an (LS)TTL input pulldown resistor.

 

[Roff]

audioguru,

You're right, the data sheet for the 4082 has the IOL
at 0.5 ma at about 25C.

The data sheet for the SN74LS393N says the IIL for the
clock pin is -1.6 ma max @ 0.4 volts, but no minimum
or temperature is stated.

My power supply is at 5.04 volts for both VCC and VDD.

Any theories why it doesn't work with the diode?

Thanks,
Jerryd

I told you why it won't work with the diode. If you didn't understand, please get back with questions.

 

[Roff]

IOL for the 4082 is irrelevant. IOL will not flow through the diode, because current cannot flow through a diode in that direction.
See my previous post also.

BTW, I'm not really a little boy. Perhaps i should change my avatar back to No BS.

 

[jerryd]

Electro Tech forum,

One more piece of data on this problem is the other
2 pins the 4082 is driving are the 2 reset pins of
a 74LS393.

The data sheet states that these pins have an IIL of -0.4 ma.

No problem driving these pins.

Jerryd

 

[jerryd]

Electro Tech forum,

I'll change the counters to 74HC393 and post the results.

Jerryd

 

[MrAl]

audioguru,

You're right, the data sheet for the 4082 has the IOL
at 0.5 ma at about 25C.

The data sheet for the SN74LS393N says the IIL for the
clock pin is -1.6 ma max @ 0.4 volts, but no minimum
or temperature is stated.

My power supply is at 5.04 volts for both VCC and VDD.

Any theories why it doesn't work with the diode?

Thanks,
Jerryd

Hello there,

It's not good practice to try to drive TTL of any kind with a diode anyway. The input or output requirements get stretched too much and you could easily lose noise immunity...something you REALLY dont want on a clock input.

The right way to do this is with a standard TTL buffer or even an inverter as it doesnt look like it will matter (double check that in your app though).
Second choice would be an NPN bipolar transistor or even mosfet perhaps. You would still need a pullup on the input to the clock (1k good) and then connect the collector to the 1k to drive the input, with emitter to ground, feed the base with a 2.2k resistor from the output of the CMOS.

You'll note that clock inputs are often more fussy than regular inputs.

 

[Roff]

Hello there,

It's not good practice to try to drive TTL of any kind with a diode anyway. The input or output requirements get stretched too much and you could easily lose noise immunity...something you REALLY dont want on a clock input.
The right way to do this is with a standard TTL buffer or even an inverter as it doesnt look like it will matter (double check that in your app though).
Second choice would be an NPN bipolar transistor or even mosfet perhaps. You would still need a pullup on the input to the clock (1k good) and then connect the collector to the 1k to drive the input, with emitter to ground, feed the base with a 2.2k resistor from the output of the CMOS.

You'll note that clock inputs are often more fussy than regular inputs.

A transistor inverts. A diode doesn't. That's a problem.
All he really needs to do is replace the diode with an OR gate.

EDIT: Or switch to 74HC393.

Another EDIT: I said that MrAl's inverting transistor is a problem. I just realized that clock polarity is irrelevant for this circuit.
Sorry, MrAl!

 

[Roff]

Electro Tech forum,

One more piece of data on this problem is the other
2 pins the 4082 is driving are the 2 reset pins of
a 74LS393.

The data sheet states that these pins have an IIL of -0.4 ma.

No problem driving these pins.

Jerryd

You don't have a problem driving the resets because they don't have the diode and the resistor.
Your circuit should work using 74HC393.

 

[Roff]

Here is the circuit with the OR gate added.

 

[MrAl]

A transistor inverts. A diode doesn't. That's a problem.
All he really needs to do is replace the diode with an OR gate.

EDIT: Or switch to 74HC393.

Some apps do not become affected by an inverted clock, especially when the clock pulse is narrow. If you read back in my post you'll see i mentioned that he would have to check that effect out to make sure an inverter doesnt bother the timing enough to cause a problem.
If one transistor does not work then two will anyway.

I do like the idea of an 'or' gate too though, or a 'nor' gate with a second section used as an inverter.

If for some reason a diode does have to be used, then i would recommend working with a Schottky diode with a lower forward voltage drop.

 

[Roff]

Some apps do not become affected by an inverted clock, especially when the clock pulse is narrow. If you read back in my post you'll see i mentioned that he would have to check that effect out to make sure an inverter doesnt bother the timing enough to cause a problem.
If one transistor does not work then two will anyway.

I do like the idea of an 'or' gate too though, or a 'nor' gate with a second section used as an inverter.

If for some reason a diode does have to be used, then i would recommend working with a Schottky diode with a lower forward voltage drop.

I edited post #16 as you were posting. I acknowledged that clock polarity is immaterial, and apologized.