From InverterVDD... Those PSpice-provided inverter symbols utilize "hidden" power pins.
In the USB ASIC there weren't actually full 74HC04 equivalents, just single-stage min-geometry inverters... going on into the ASIC... i.e. no heavy loading.
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
RTFM.
electrical characteristics over recommended operating free-air temperature range
?ICC(*), One input at 0.5 V or 2.4 V, Other inputs at 0 or VCC,
2.9mA *max*.(*) This is the increase in supply current for each input that is at one of the specified TTL voltage levels, rather than 0 V or VCC.
Irrelevant. There was a very specific problem, and HCT/AHCT is a correct solution.
Very few people can accept their own mistakes, unfortunately. Life would be much better otherwise.
-- WBR, Yuriy. "Resistance is futile"
Yuriy,
You are correct. Here's more supporting documentation.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ AHC/AHCT Designer's Guide
September 1998, revised February 2000
Starting on page 19:
To process signals having a swing of only about 3 V in a system with a supply voltage of 5 V, the TTL-compatible SN74AHCT14 Schmitt trigger is available. This device has the same switching characteristics as the previously described Schmitt trigger, except that appropriate circuitry shifts the switching thresholds into the region of the commonly used TTL-voltage levels. Figure 5 shows the transfer function of such components.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ SN54/74HCT CMOS Logic Family Applications and Restrictions
Power Consumption of HCT Circuits
The threshold voltage of a CMOS circuit is determined by the geometry of the input transistors. These transistors are designed to sink the same input current at the required threshold voltage. The resulting voltage at the output is equivalent to 50% of the supply voltage VCC. For an HC circuit, the channel width of the p-channel transistor of the input is approximately twice the value of an n-channel transistor. The purpose is to make both transistors have the same current characteristics, thus making the threshold voltage of their input at about 50% of the supply voltage VCC. This circuit area has been modified for HCT devices: the n-channel transistor is about seven times wider than the p-channel transistor (see Figure 6). This shifts the threshold voltage in a way that it amounts to 30% of the supply voltage. At a supply voltage VCC = 5 V, the threshold voltage is VT = 1.5 V, similar to the threshold voltage of TTL circuits.
Figure 6. Input-Stage Structure of HC and HCT Circuits
Figure 7. Supply Current as a Function of the Input Voltage
Figure 8. Current Consumption as a Function of Frequency
For frequencies above 5 MHz, this effect is of secondary importance, since current consumption is then determined primarily by the power required for reversing the charge of the load capacitance. Moreover, the increase of current consumption for devices driven by TTL levels is much lower in practice because TTL circuits supply a typical voltage swing that is significantly higher than the data sheet value used for the measurement in Figure 8.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Regards,
Mike Monett
Antiviral, Antibacterial Silver Solution:
Alison,
is a explanation of the different logic level voltage translation methods and ICs.
-- WBR, Yuriy. "Resistance is futile"
I think that in the non-ASIC world where adding extra transistors is not free, it is less frowned-upon to allow CMOS logic to draw continuous supply current. I don't know of any chips failing due to doing this, with the exception of 74AC series when running from 5V and biased at the threshold, which really does draw a lot of current.
OnSemi do in fact recommend the use of HCT14 devices to receive signals from LSTTL chips (see for yourself.... perhaps their marketing dept needs re-education):
The datasheet "Recommended Operating Conditions" explicitly specifies that there is no minimum input slew rate, and the supply current with one input held at 2.4V is specified as 2.4mA (typ), implying that it is at least an allowable operating condition. Certainly 2.4mA is far below the specified maximum IDD for the part.
It seems to me that the HCT14 will work, and not fail prematurely, when one inverter is driven by a 3.3V input signal. Perhaps you know better, and if so I'd like to hear about it.
I think your circuit with no steady state current consumption would be ideal if one were designing an ASIC, but the intended application is not battery powered, and the HCT parts are cheap, available from several sources, less likely to be discontinued than an exotic level shifter IC, and result in lower component count than using an additional CD4007 (which would probably be too slow anyway when it's required to go to 15MHz). BTW why is the simulated waveform from your CD4007 circuit so fast? Perhaps you don't have PCB parasitics in your simulation. Most manufacturers of 4007 parts seem to specify about 60 - 75ns maximum propagation delay at 5V VDD. I wonder if anyone makes a CD4007 type thing in a more modern process.
Chris
If you're comfortable with that "overlap" current, go right ahead.
I still consider it bad engineering practice but, then again, I'm presently designing a chip (analog, but LOTS of logic) with a total average power consumption specification of 6uA. Overlaps like that would kill the consumption.
I do note that the TI app-note does recommend the Schmitt. But it depends on the Schmitt design whether you still have the overlap current or not.
If 'HCT04 were a good general solution to 3.3V to 5V translators, wonder why they make true translator chips like the Maxim MX3xxxx to name just one? For their health? I doubt it.
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
Hi Rickman,
It's looking alot tidier now. Should keep me going with the firmware development until the PCBs appear.
The board with the chip on it has most of what's required SPI side condensed down. There's a board under that takes care of getting the Vdd/Vss to the all of the pins (PICs are picky about that). And the ribbon cable goes off to a kind of hybrid SCSI bus implemented on a 68000 processor DMA bus.
Thanks again for your help :-) Thanks to all of you :-)
Alison
Will you cut down the bullshit about ASICs, USB and nanoamperes. We are talking about interfacing the 3.3V CMOS input level from a flash card to
5V level input of a dsPIC. It is perfectly done with a simple 74HCT gate, and this solution you can find in any ABC book on electronics.BTW, where did you get the 2.4V? JFYI: for 3.3V CMOS level and 5Vdd, the consumption is just 0.3mA. This is a perfectly documented mode of the operation, RTFM.
I don't care. It is absolutely irrelevant.
You see, grandpa, what you are doing is too old fashioned.
There is no such thing as Russia. There was, there is and there will always be the USSR. Unfortunately.
Very well could be. The jews of Israel are actually changing to arab-like nation.
Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
If you run into problems, then as I live near Cambridge (work near there too), I might be willing to help you out - think of it as a xmas gift or whatever.
Cheers
PeteS
There is no way to argue with a punk. Don't go away mad, just go away.
When your accumulated income exceeds $5 million, *perhaps* I'll deign to listen to you ;-)
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
isn't High typically specced as anything above 2/3 VCC.... (that'd be 3.33v)
What voltage are they running the DSPIC ?
if you power the PIC from 3.3V it'll probably work just fine
-- Bye. Jasen
To give yourself some more margin, look at adding some parallel C to the series resistor (same peaking/matching princple as a scope probe), on those lines that need to do 5V to 3V.
What you are making is a capacitive divider (high frequency) that matches the resistive divider (low frequency), and it allows the resistor values to increase without a speed penalty (for lower power). eg If the total load shunt C is 20pF, you'll need appx 40pF.
Avoid transistors, they will be too slow.
-jg
.. When driven to the rails ?
Interesting circuit; do you have the plots of Icc vs Vin, increasing and decreasing ? And also Icc into/out of 5V and 3.3V rails ?
Limits of Vcc delta's ?
-jg
There are a few other things you might want to consider. I don't know how many of these you plan to make or how they will be used. But anytime you have external signals, it is a good idea to protect them from ESD. If you can work with a real PCB design (made by a PCB maker), you can use a part that might be better. Here is one I found at TI.
It provides 15 kV ESD protection and will do the level translation without the resistors and messy circuitry. It does not come in a 0.1" DIP, but it is available in SOIC which is 0.05" pin spacing, large by today's standards. You should be able to get a few as free samples. All things considered, something like this is the route I would take. I don't know how SD card I/Os are typically protected, but this is something you might want to take seriously.
If you want to have a PCB made for you inexpensively you can go to Olimex which I think is in Eastern Europe.
Oh, be sure to use lots of power and ground plane area, flood fill if you go two sided. This provides a good high frequency capacitor that minimizes noise on the power planes.
Let us know how your design works out!
Maybe I don't undestand what you mean by "copperclad". To me that is the same as PCB. Are you talking about mouting components to a pre-etched general prototype board or a PCB that has *not* been etched at all, or something else?
Not quite to rails.
See (updated)....
V1 = 5V, V2 = 3.3V
I don't know an answer for that. This presentation was just a lash-up. My actual ASIC implementation uses single-stage inverters instead of the 74HC04's, so my sizing ability allows very low overlaps... it's basically a form of hysteresis.
I'd guess that wildly disparate power supplies would present some slew issues and give more overlap current... but I'd hazard a guess that
1.2V -> 5V would be easy....Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
Maybe I'm dense but isn't that that pretty much the intended application for HCT parts? specified in the datasheet down to much extra current consumption when going to limits of Vi and Vcc
-Lasse
"HCT" parts were designed to receive signals from TTL parts also operating at VDD=5V.
The shifted threshold was done to minimize timing skew when making the translation.
Over time they've come to be misused as logic level translators, even though there are specific parts for such purposes.
Unfortunately many lurkers here presume that "getting away with it" is good engineering practice :-(
Me, I have to deliver good stuff every time... otherwise I'm unemployed.
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
look at olimex.com they are pretty cheap, 160x100mm two layer pcb with solderstop and silk for something like 33$
-Lasse
The venerable Alison etched in runes:
. . .
Not at all. Have a look here:
and go to the 'Plot & Go' service. They'll do one-off prototypes very cheap and good quality too.
-- John B
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