Electronic – 74HC595 with no latch pin

shift-register

If I use the 74HC595 output shift register, and I don't use the latch pin, would it still work as shift register, like the 74HC164, that doesn't have that pin at all?
The shift register is going to be used to reduce the output pins needed to drive the colums on a switch (pushbutton) matrix.

Best Answer

No. Both SRCLK and RCLK are edge-triggered, so if you hold RCLK at a fixed level, the outputs will never change.

You'd have to connect RCLK and SRCLK together in order to get the functionality you want, but note that this creates an extra clock of delay from the serial data input to the first output, as compared to the '164.

Related Solutions

Electronic – arduino – Sharing Clock and Data for 74HC595 and CD4021B Shift Registers

If I may answer my own question. It should work. Here's how with arduino over SPI.

void setup() {
    //Setup SPI
    SPI.begin();
    //Setup CD4021
    pinMode(CD4021_PSC_PIN9, OUTPUT);
    digitalWrite(CD4021_PSC_PIN9,HIGH);
    //Setup 74HC595
    pinMode(HC595_LATCH_PIN12, OUTPUT);
    digitalWrite(HC595_LATCH_PIN12,HIGH);
}

void loop(){
    //Shift out to 74HC595
    digitalWrite(HC595_LATCH_PIN12,LOW);
    SPI.transfer(BYTE_DATAOUT);
    digitalWrite(HC595_LATCH_PIN12,HIGH);

    //Shift in from CD4021B
    digitalWrite(CD4021_PSC_PIN9,LOW);
    BYTE_DATAIN = SPI.transfer(0x00);
    digitalWrite(CD4021_PSC_PIN9,HIGH);
}

Learned from here, from someone who has done this exact implementation before.

There is no ‘latch pin’ on MC74HC164A shift register

The '595 has two sets of latches; one is an input latch that receives the external data serially, and the other is an output latch that receives the data in parallel from the input latch.

The '164 has no discrete output latch; all data is output in parallel as it is received serially. The timing diagram on page 7 shows this.

Best Answer

Related Solutions

Electronic – arduino – Sharing Clock and Data for 74HC595 and CD4021B Shift Registers

There is no ‘latch pin’ on MC74HC164A shift register

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