![]() ![]() If the '595 is driving LEDs, you probably won't see it (might see a faint glow on LEDs that are off if the '595 is updated regularly. 74hc165 hardware spi atmega code#I just reworked it using a pair of 23s17, and it simplifies the schematic, and greatly cleans up the board layout (and takes less room.) It increases the code complexity, but I think it will be worth it.Īt the same time, the corrupted data would not be at the HC595 outputs very long, just 20 clock cycles or so, 1-2uS, so there may or mat not be a noticeable change at the system level. Thanks for the reminder! I'm working on a project where I was trying (somewhat unsuccessfully) to make a true SPI interface out of shift registers. But it's hard to implement a true SPI interface with shift registers. Now, sharing the bus is not part of the OP's problem statement, so this may not be an issue for him. This could have very bad effects depending on what's connected to those outputs. That value will get latched to the outputs for the time between the two latch pulses. That's not a problem since the shift register still has the previous value from the last transfer.īut if there are other devices on the SPI bus, when the latch is pulsed that first time, the '595 shift register will have whatever was the last byte sent to the other device. This will once again latch the '595 shift register to its outputs. Then, when you do the next transfer, the first thing that happens is the CS line is pulsed. If there are other devices using the same SPI bus (with a different CS) this will fall apart. I questioned this at first, but as I think about it, it looks like it will work, but only if these shift registers are the only thing on the bus. You can't do this with shiftin() and shiftout() since they are unidirectional, but you can do it with SPI.transfer() or by manually bit-banging the interface.ĬrossRoads: digitalWrite (latchPin, LOW) ĭigitalWrite (latchPin, HIGH) // captures data on '165 inputsĭataIn = SPI.transfer(dataOut) // read in from '165 while writing out to '595ĭigitalWrite (latchPin, HIGH) // move data to output of '595 When done, the internal shift register holds the input byte. ![]() As the bits are shifted out MOSI from one end, it is shifting in bits from MISO on the other end. You start by loading the register with the output byte, then the hardware starts clocking the shift register. ![]() The SPI hardware in the processor has a shift register. Sounds like he's trying to make a normal SPI bus, which always operates bidirectionally, although often we ignore the input or output side of a particular transfer. ![]()
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