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CD4541B: Functionality, setup and input questions.

Diana Kraemer
Prodigy 50 points
Part Number: CD4541B
Other Parts Discussed in Thread: CD4017B

Hi, 

I'm using the CD4541B to create an automotive timer. I know the limitations of the clock and am planning on using an external oscillator for accuracy. However, to verify the functionality of the chip, I've been experimenting with shorter times. 

What I want is for the device to stay on for X time after the vehicle is turned off. I am planning on setting this time with R_tc. 

My current set up: 

Vdd=13.1 V (generally 12, because automotive)

A, B: Vdd for count is 65536. 

Pin 5: Vdd for auto reset disable. 

Pin 6: Initially ground, then pulsing to Vdd for timing applications. 

Pin 9: Vdd for initially high after reset

Pin 10: Ground for single transition mode. 

With R_set=100K, R_tc=47K, C_tc=.1uF (I'm currently limited with my capacitor selection). My calculated oscillator frequency from the data sheet should be ~100Hz. And it's my understanding that Run Time=Count/oscillator frequency, which should give me 65536/100=about 10 hours. I know this is incorrect because I've read forums saying that even 30 minutes is impossible with the built in RC and I'm only getting 2.3 minutes with this set up. My count is 14,500 based on this information. 

So, I'm confused and here are my questions: 

1.) What is the count actually? My A and B are tied to Vdd, why is my count not 65,536? The data sheet says with Q=initially low, after 2^N-1 counts the timer will go high. I can deal with that, but why the discrepancy between 2^N in the chart and 2^N-1 in the description? 

2.) Is my understanding of the timing calculations correct? If not, what am I missing? Because if f_clock is 1kHz, then shouldn't the max time be 1 hour? 

3.) On page 4, the data sheet suggests that the limits of an external clock is 6MHz. Is this true? 

4.) Should I just get a different chip? This is extremely frustrating and I'm very confused. 

  • 0
    TI__Expert 6680 points

    Hi Diana,

    Sorry for any frustration, I'll see if I can help.

    On first glance, it seems like you're within spec; however, I think the issue is that the formula doesn't necessarily apply for f < 1kHz. Have you tried picking a capacitor that would give you f~= 1kHz and seeing if the timing works out?

    My suggestion would be to keep the counting of the CD4541B to ~1 pulse per hour, and counting 9 pulses with a CD4017B.

    Thanks,

    Chad Crosby

  • 0 in reply to Chad Crosby
    Prodigy 50 points

    Hi Chad, 

    Thanks for the response. I have not tried that suggestion but it's a good idea. 

    The device does work at f<1k, just not reliably. My 2 hour timer lasted about 2.2 hours, so I thought using an external oscillator would be a good solution. 

    I'm not sure what you mean by using the CD4017B. Are you suggesting the 4541 be the clock for the 4017? 

  • 0 in reply to Diana Kraemer
    TI__Expert 6680 points

    Hi Diana,

    You are correct. I managed to find the circuit I was thinking about here.

    Thanks!

    Chad Crosby

  • 0 in reply to Chad Crosby
    Prodigy 50 points

    Thanks Chad. However, that's not really what I'm looking for. The decade counter will change outputs-so initially it will be on output pin 1 and then after an hour it'll switch to output pin 2 and so on. I need the device to stay on and constant for a period of time, but then turn off after that time has expired. 

    Switching outputs brings the potential of dropped voltage and/or spikes that I can design around with a cap and diode, but why bother? Unless the 4151 will have difficulty with a slow (~4Hz) clock input on pin 3? I thought about chaining two 4151s, but then how accurate is output frequency vs the RC network. 

    It's easy enough to do with an analog setup, but that's not providing the accuracy that I want. 

  • 0 in reply to Diana Kraemer
    TI__Expert 6680 points

    Hi Diana,

    The tricky thing about any RC oscillator is that they tend to not be as accurate as other types of circuits, such as XOSCs and timers on microcontrollers. If accuracy is what you're most concerned about, I'd recommend switching to one of those other options.

    I think the main issue you might see with this circuit is some retriggering because of the slow rise time (see below). This is most likely why you're seeing 2.3 minutes as your output time instead of your desired 10 hours. If you still do decide to go with the CD4541B as the oscillator, I still recommend using the CD4017B to count 10 pulses from the clock every hour - this will try mitigate any retriggering on the circuit by increasing the oscillation frequency and thus the input rise time.

    Thanks!

    Chad Crosby

  • 0 in reply to Chad Crosby
    Prodigy 50 points

    Thanks for the info link, I'll definitely check that out.

    I don't need 10 hours, only 2. The 10 was my attempt to explain my understanding of how the chip operates.

    I'll look at some of the other things you suggested. It's a simple (well, supposed to be) timing device to keep equipment running after a vehicle is turned off. The analog ones we use work, but aren't very accurate. The dip switch we use is labeled "1-3 hours" so I thought I'd redesign it. Good learning experience but turning out to be more involved than I thought.

  • 0 in reply to Chad Crosby
    Prodigy 50 points

    Hi again, 

    I'm wondering what the output of the circuit should be upon power up. Because no matter what configuration I have it in, as soon as I turn power on, the output (an LED) turns on. 

    Thanks! 

  • 0 in reply to Diana Kraemer
    TI__Expert 6680 points

    Hi Diana,

    We don't specify how this device will behave at startup with Pin 5 (Auto Reset EN) pulled high. This is a latched device, so the following FAQ applies here: [FAQ] What is the default output of a latched device?

    I'd recommend toggling the master reset after startup, or pulling Pin 5 low.

    Thanks!

    Chad Crosby

  • 0 in reply to Chad Crosby
    Prodigy 50 points

    Thank you so much. 

    I do toggle the master reset after start up to begin the timer. It was working great yesterday, but I moved pin 5's wire to toggle instead of 6 and I think that fried a chip because it got hot quickly after fixing the mistake and repowering. 

    Maybe I fried my backup, too? Shouldn't I see an oscillating voltage on R_TC, C_TC and/or R_SET? With my current configuration (5 is high, 6 is initially low then toggled to high and low for a trigger, 12 and 13 are high, 10 is low and 9 is high), I've got 84mV on C_TC but nothing on the resistors and the output doesn't turn off. 

  • 0 in reply to Diana Kraemer
    TI__Expert 6680 points

    Hi Diana,

    The chip should not heat up during normal operation, and is usually a good indication that absolute maximum ratings have been violated.

    Thanks!

    Chad Crosby