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use of cascade 74HC595 with atmega 8

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selva murugesan

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hi

i am doing the project in atmega 8 using codevision avr .In that i need 12BIT DAC. for elimination large number of PINs , i am using 74HC595. My idea is send the 12 bit values to the 74HC595 via single I/O pin.then the 74HC595 deliver inputs (12 bits ) to the DAC.

in this process i succeed partially, i cant do proper shift adjustment in 74HC595. In total 12 bits , i can shift and send the last 8 bits from LSB properly . remaining bits gets shuffle.


can any one help me?

thanks in advance
 

Hello selva,

can you describe your wiring between AVR uC and your 74HC595 chain or can you please show your schematics?

Regards
 

thanks asim

here i added one paint file
 

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Hello Selva,
What I understand from your scheme is you want to shift 12 bits to provide input to DAC which is connected with two parallel outputs of your 74hc595 ICs.

The link that alexan has provided is quite expanatory. Please notice that ST_CP signal must be asserted (from low to high) and then de-aserted (from high to low) to transfer serial register contents on the 74595 parallel outputs. Since you are using two cascaded serial shift registers which both have common ST_CP signals and in your scheme you are driving the serial data input of your second shift register with D7(MSB) of first shift register, therefore to transfer contents to your second high order shift register you will need ST_CP signal to be asserted every time you shift a single bit. In fact you should combine both the Clock (SH_CP) and ST_CP signals togather to make them a single wire being driven as clock from your microcontroller.

If your dac does not has any strobe or write enable then you may have problem that while a 12bit word transfer is in progress you will see errornous values apperaring at the DAC output.

Regards
 

hai asim,

thanks. i understood the problem of that circuit.
 

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