Comments on: Chapter 4: Interfacing Electronics

By: Derek

Derek — Mon, 21 Mar 2016 03:11:25 +0000

In reply to Shawn. Hi Shawn, First the NAND Gate: A NAND gate has an output of 0 when both inputs are 1, but 1 in all other cases. Therefore, as A is high the output is dependent on B. If B is high then the output of the NAND gate is low. Second the FET: A low input into the gate of the FET results in an output of 5V as the FET is turned off and so the output is tied to the 5V supply (through the 10k resistor). Similarly, when the gate input is high it turns on the FET and the voltage at the output is 0V as the current that results from the 5V supply and the 10k resistor sinks to ground through the FET (drain/source). This is one of the more complex circuits in that chapter, so you are doing very well if you can get a handle on this one! Kind regards, Derek.

By: Shawn

Shawn — Sun, 13 Mar 2016 05:14:09 +0000

Hi Derek,
I have a question about figure 4-26. When B is high, the output of NAND gate should be low, and the FET acts just like switch opened. So the Out will be ~5V, but it doesn’t make sense. Did I miss something? I just feel overwhelmed in this chapter.
Many thanks.
Shawn.

By: Derek

Derek — Sat, 21 Mar 2015 02:21:56 +0000

In reply to Gordon. Hi Gordon, I agree that the way you have tested the SN74HC03 is perfectly correct. One thing I noticed in the datasheets that is different is that the SN74HC03 gives the output state as explicitly High/Low, whereas the M74HC03 lists the outputs as High-Impedance(Z)/Low. There isn't much detail in the SN74HC03 datasheet but my guess is that both ICs have an entirely different internal configuration and behaviour, despite both having an open-drain output. Very interesting! Derek.

By: Gordon

Gordon — Fri, 20 Mar 2015 20:26:38 +0000

Hi Derek,
Thanks for the quick response! After posting my comments yesterday I thought that I had better test the circuit myself. I used an Arduino Due with a slightly modified Blink program to generate a 3.3v square wave. As well as the square wave the Due was used to supply 3.3v power to the 74HC03. (The exact chip I used was a Texas Instruments SN74HC03N) I connected a 10k ohm resistor, as a pull up load, between the open drain output and the positive of a variable DC power supply. The negative was connected to a common point. I set the variable supply to 3.3 volts and monitored the output square wave on a ‘scope. The voltage was switching between 0v and 3.3v, as expected. As the variable voltage was increased, the output tracked the supply rail until it reached 4 volts. After this point the output voltage did not change with increasing supply voltage. I repeated the same test with an Arduino Uno. This time the supply power, to the 74HC03, was 5 volts. Again, output voltage tracked the supply until it reached 5.7 volts. Once more it did not change with increasing voltage on the variable supply.
It seems strange, but there must be changes between 74HC03s made by different manufacturers. Finally, if you have the time, I would be more than happy to send you my 74HC03 for testing.
Kind regards,
Gordon.

By: Derek

Derek — Fri, 20 Mar 2015 14:04:11 +0000

In reply to Gordon.

Hi Gordon — thanks for your support! That’s an interesting point. I found many textbooks to be quite dated in their treatment of circuits (op-amps in particular have evolved hugely compared to their textbook description!) so I built every circuit in the book. The plot on the RHS is the actual output from a M74HC03B1R wired as per the figure. I checked the datasheet for a few different 74HC03s and they state that “The 74HC03 have open-drain N-transistor outputs, which are not clamped by a diode connected to Vcc.” The figure on the LHS is somewhat simplified (as is the datasheet) and the diode that is displayed is referred to in the M74HC03B1R datasheet as an “output protection diode”. It appears that its presence does not appear to clamp the output at all — the exact reason for this is not clear to me and it will require further reading as there is not enough information in the datasheet alone. To confirm my sanity, I connected a M74HC03B1R to the Analog Discovery as per the figure and the output is exactly the same as in the plot on the RHS (i.e., sitting nicely at 4.924V for 4.980V/3.323V supplies). Hope that helps, Derek.

By: Gordon

Gordon — Thu, 19 Mar 2015 19:49:35 +0000

Hi Derek,

In Chapter 4, Page 139, Figure 4-26, the 74HC03 would appear to have clamping diodes on pin 11 output. Would this not limit the max output voltage to Vcc plus the diode drop (3.3v + 0.7v)? I think this should still be okay to feed a TTL gate but Vout won’t reach more than 4 volts.

Really enjoying your book! Good mix of theory and practical examples.

Regards,
Gordon.

By: Derek

Derek — Sun, 08 Mar 2015 12:08:06 +0000

In reply to Hyusein Gyulyustan. Thanks Hyusein, Yes, that should be Figure 4-28(b). I'll add it to the list of errata, Derek.

By: Hyusein Gyulyustan

Hyusein Gyulyustan — Sat, 07 Mar 2015 19:32:33 +0000

On page 114: “The circuit example used in 4-27(b) could be used to generate PWM signal…”, I think you mean “4-28(b)” instead of “4-27(b)”.

By: Derek

Derek — Wed, 21 Jan 2015 20:54:40 +0000

In reply to Michael K Johnson. Thanks Michael, you are correct -- I will add that to the list of errata. Thanks, Derek.

By: Derek

Derek — Wed, 21 Jan 2015 20:52:51 +0000

In reply to The Reynolds. Thanks for that -- you are perfectly correct. The book was always going to be printed in grey scale but my brain was still working in color! Yes, the blue multi-turn potentiometer is on the bottom left of the figure. I'll add a color copy of the figure to the web page. Thanks, Derek.