Electronics > Microcontrollers

testing MC146818

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Wawavoun:
Hello there,

I have a MC146818 RTC which did not work !

With a HC49-S 32.768 KHz crystal I have no oscillations on pin 3 and on pin 21 (should be 31.768/4 if pin 22 FS is grounded).

My purpose is to find if the problem come from the chip or from the quartz ???

I have other and different crystals (cylindrical 3x8) and want to test everything on a breadboard to be more sure from where the problem come...

What is the minimal connections to do to get the MC146818 alive with clock output to pin 21 ? VSS, VCC, FS, CE, crystal circuit to pin 2-3 are sufficient ?

Regards.
Philippe

srb1954:

--- Quote from: Wawavoun on December 03, 2021, 08:45:53 pm ---Hello there,

I have a MC146818 RTC which did not work !

With a HC49-S 32.768 KHz crystal I have no oscillations on pin 3 and on pin 21 (should be 31.768/4 if pin 22 FS is grounded).

My purpose is to find if the problem come from the chip or from the quartz ???

I have other and different crystals (cylindrical 3x8) and want to test everything on a breadboard to be more sure from where the problem come...

What is the minimal connections to do to get the MC146818 alive with clock output to pin 21 ? VSS, VCC, FS, CE, crystal circuit to pin 2-3 are sufficient ?

Regards.
Philippe

--- End quote ---
Checking the operation in a breadboard may not be all that successful as the the stray capacitances inherent in breadboards may upset the sensitive oscillator circuitry. Generally is is advisable that all other tracks be kept well away from the crystal and its associated components. In any case strictly follow the recommended circuit configuration in the data sheet and ensure that the crystal load capacitances are as specified by the crystal manufacturer.

The CE, CKFS and RESET pins should be tied high and other input pins AS, DS, R/W, PS and AD0-7 should be tied low for basic oscillator operation producing a 32768Hz signal on CKOUT. The SQW output will not be activated until the chip registers have been suitably programmed.

Benta:
First time I've heard about a 32.768 kHz crystal in HC49.
Are you sure it's not 32.768 MHz?

Wawavoun:
Hi,

I have also think to that... I will test the quartz with a simple double 74ls04 gate and see...

SiliconWizard:

--- Quote from: Benta on December 04, 2021, 04:57:01 pm ---First time I've heard about a 32.768 kHz crystal in HC49.
Are you sure it's not 32.768 MHz?

--- End quote ---

I second that. I find this fishy. And if it's indeed a 32.768 MHz crystal, that would explain why the MC146818 doesn't oscillate at all.

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