DS1302 resistor up-date

hmlittle59

I saw an drawing of a DS1302 circuit on the forum and it had a 1k ohm resistor in series with pin six(6) and MC. I don't remember seeing this in the original circuit and I have it in my design along with a 74HC595 that has the "Latch" pin attached to "Grd" thru a 10k ohm resistor. Should I attach the appropriate pin from the DS1302 thru a resistor or this same resistor? This is my 4th spin and it will be SMT and I want everything correct.

I guess the main question is should I just attach it to the same resistor or a separate one all together?


thanks for any help.

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I HAVE LEARN SO MUCH...BUT STILL KNOW SO LITTLE!!!

hmlittle59

Carl Hayes

Pin 6 on the DS1302 is the I/O pin (on the 8-pin packages anyway).· I assume you mean that there's a 1K resistor through which this pin is tied to an I/O pin on a microprocessor.· If the micro and the DS1302 are operating with different Vdd, this would protect the lower-voltage one.· If they're operating on the same Vdd, it's not necessary and I'd leave it out.

By "latch" on the 74HC595 I assume you mean the output-enable, pin 13, active low.· If I were tying this to ground, I'd just tie it to ground, no resistor.

But maybe you mean pin 12, the latch clock.· You don't want to tie that to ground, 'cuz it's got to transition or you get no parallel output.· Of course, if you want only serial output from the 595, you can take it from the unlatched Q7 output (Pin·9) and do whatever you want with Pins 12 and 13 as they affect only the latched parallel outputs.· For Q7 the 595 has two outputs, latched on Pin 7 and unlatched on Pin 9.

If the 595 is feeding the DS1302 and the DS1302 is running on 3.3v while the 595 is on 5v, I'd put in a resistor.· It's got to be a separate resistor, though.

This response is based on my assumptions about the circuit you're trying to describe.· It would be better to attach an image of a schematic diagram so I could try to avoid saying anything dumb.

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· -- Carl, nn5i@arrl.net

Post Edited (Carl Hayes) : 2/20/2009 5:47:50 PM GMT

Beau Schwabe

hmlittle59,

Voltage level translation is not the only reason for placing a series resistor on an I/O pin. It's also placed there to prevent a potential data collision if both devices are transmitting a logic level and they don't agree with one another. i.e. One device is transmitting a HIGH while the other device is transmitting a LOW.

As Carl suggests however, you will receive a more accurate answer if you post a schematic of what you are trying to describe.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Chris Savage

I believe the circuit in question was mine (I use a 1K resistor on the DS1302) and Beau’s reply is spot on in that since the pin is bi-directional, it is possible for a programming mistake to cause a situation where the I/O pin may be damaged. To prevent this the 1K resistor is cheap insurance to protect both chips. Take care.

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Chris Savage
Parallax Engineering

hmlittle59

Hello All,
Thanks,
Yes Chris S. this is from your design you suggested I look at with the 74HC595. I connected the DS1302 and 74HC595 together as suggested also. Right now the 75HC595 only needs to turn ON/OFF of RED/GREEN LEDs. I daughter boarded the LEDs, 74HC595, 10k ohm to "GRD" on the BOE, and used jumper wires to my board and got it working with some other suggestions from this forum, then saw the up-dated DS1302 layout.

1) Could I just TAP off the Positive side of the current 10k ohm resistor on the 74HC595 pin 12 IC...(easy, easy, easy, easy!!!)
--- OR --
2) Just add the 10k ohm in series from BS2 and move on...(Easy...do able).

PS:So I'm clear, the main reason for the 10K is to protect the IC pin from those of us that may install/remove...CHANGE program I/O pin direction. But once my program is up and running in a design the resistor will not be needed as the program flow should only be from the BS2(MC) to the DS1302 chip(setting, changing time).
Is this close...almost close in my understanding. I'll read Carl's POST 5 or 6 more times also...DEEEP!!!
Don't know I can come close to attaching a schematic to this text. Sorry

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I HAVE LEARN SO MUCH...BUT STILL KNOW SO LITTLE!!!

hmlittle59

Chris Savage

Was the resistor 10K or 1K? It should be 1K for the I/O pin in series. 10K would be used for pull-up/pull-down on lines.

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Chris Savage
Parallax Engineering