74590 IC pins explained to beginning circuit builder

Thread Starter


Joined Jan 19, 2018
I am working on a project and I need to design a binary clock within it. I purchased several 74590 8 bit counter chips. I am familiar with the 7493 4 bit counter and thought that the 74590 counter would be similar but Im getting nonsense outputs when I power the chip, ground the chip, connect a driving 555 timer to the CPC and connect Q0-Q7 to output LEDS. Can someone explain the pin connections to the 74590 chip (data sheet)? Or explain what could be going wrong?

Also what is an "output register; 3 state" and how will this effect my binary clock?

I'm just beginning to get into circuits and appreciate your clarity and thoroughness. Thank you!



Joined Oct 2, 2009
You posted a link to the data sheet. Did you read the data sheet?
Did you see Table 3 - Function Table?

There are a bunch of inputs. How did you connect these?

14 /OE
13 CPR
12 /CE
11 CPC
10 /MRC

Thread Starter


Joined Jan 19, 2018
Wow I completely missed table 3.

Looking at it now I hooked up the following:
14 /OE low
13 CPR ?????????? - what is a register?
12 /CE low
11 CPC is the output of the 555 timer
10 /MRC high

I'm now getting all of the LED outputs on but still no counting.


Joined Mar 30, 2015
You should use current limiting resistors for the LEDs, even if they're just for temporary indicators. The outputs can source 6mA, but an LED without a current limiting resistor would clamp the output to the LED voltage.

RCO# is an output, you shouldn't tie it HIGH; it's a LOW active output for carry.


Joined Jun 19, 2012
Some simple rules-of-thumb for working with digital logic chips:

1) Power and ground pins must be connected to the appropriate supply voltage for the part.
Use a REGULATED power supply. A 0.1 uf bypass capacitor on the Vcc pin of each chip is good practice, keep this cap close as possible to the IC pins.

2) All INPUTS must connect to either Vcc, Gnd, or an other chip output, never leave them floating.

3) Never connect unused OUTPUTS to GND or Vcc., leave them unconnected.

4) Never use an LED without a resistor to monitor an output.
The LED clamps the voltage, stressing the parts while also corrupting the output voltage level.
If you try to drive another input with the pin, it might not work.

Following these simple rules will help avoid 90% of newbie troubles.


Joined Dec 29, 2010
74hc590 block.PNG
According to the block diagram above. Am I right that CPC is connected to the pulses that you want to measure and CPR is connected to the system/mcu clock?

IOW, CPR should be faster clock pulses than the CPC input pulses ?



Joined Oct 2, 2009
Allen, you have touched on an important characteristic/feature of the 74HC590.

The output of the counter does not appear directly on the output pins of the chip. There is an intervening holding register that must be clocked.

74HC590 Table 3.jpg

Table 3 shows that while the counter increments on the LOW-to-HIGH transition of CPC, the register is latched on the HIGH-to-LOW transition of CPR.

74HC590 Fig 13.jpg
Fig 13 above tells a different story. It says that the setup time between the HIGH-to-LOW transition of CPC to the HIGH-to-LOW transition of CPR is tSU.

74HC590 Table 7.jpg

tSU for Vcc = 4.5V is 30ns minimum.

This means that the CPC must be at least 30ns ahead of CPR. Hence if CPR is tied to CPC, the output of the chip will always be one count behind what is in the counter. It is an important point to note. How this affects the operation of your circuit depends on the application and how the counter is to be used.

For example, if you are designing a frequency counter, you can allow the counter to do its job of counting and only clock the output register when the count period has completed. There is no need to clock the output register faster than the input clock signal.

All of this demonstrates the importance of the saying RTFM, which stands for Read The Fine Manual.