FastShiftIn

Arduino library for AVR optimized shiftIn - e.g. for 74HC165.

Related libraries

• https://github.com/RobTillaart/FastShiftOut
• https://github.com/RobTillaart/FastShiftInOut
• https://github.com/RobTillaart/ShiftInSlow
• https://github.com/RobTillaart/ShiftOutSlow

Description

FastShiftIn is a class that has optimized code (AVR only) to shift in data faster than the normal shiftIn() function. It speeds up the shift using low level ports and masks. These are predetermined in the constructor of the FastShiftIn object.

If not an ARDUINO_ARCH_AVR or ARDUINO_ARCH_MEGAAVR the class falls back to the default shiftIn() implementation.

Since 0.3.2 read16(), read24() and read32() are added. These are experimental and not fully tested yet.

Performance

The performance of read() is substantially faster than the default Arduino shiftIn(), but not as fast as HW SPI. Exact how big the performance gain is can be seen with the example sketch. It does a comparison and shows how the class is to be used.

Time in microseconds, Arduino UNO

function	0.2.3	0.3.2
read()	19.30	20.49
read16()		41.04
read24()		62.91
read32()		83.95
readLSBFIRST()	19.04	19.92
readMSBFIRST()	19.04	19.92
reference shiftIn()	107.82	108.20

0.3.2 is a bit slower (incl. reference) than 0.2.3 but still much faster than the reference.

Interface

#include "FastShiftIn.h"

Functions

• FastShiftIn(uint8_t dataIn, uint8_t clockPin, uint8_t bitOrder = LSBFIRST) Constructor
• uint16_t read(void) reads a new value, 8 bit.
• uint16_t read16(void) reads a new value, 16 bit.
• uint32_t read24(void) reads a new value, 24 bit.
• uint32_t read32(void) reads a new value, 32 bit.
• uint32_t lastRead() returns last value read.
• bool setBitOrder(uint8_t bitOrder) set LSBFIRST or MSBFIRST. Returns false for other values.
• uint8_t getBitOrder(void) returns LSBFIRST or MSBFIRST.
• uint16_t readLSBFIRST(void) optimized LSB read(), 8 bit.
• uint16_t readMSBFIRST(void) optimized MSB read(), 8 bit.

Byte order

It might be that read16/24/32 has bytes not in the right order. Then you should use multiple calls to read() and merge these bytes in the order you want them.

Notes

• The optimizations are AVR only for now, other platforms may follow.
• The 74HC165 needs 0.1uF caps and the data and clock lines may need
  pull up resistors, especially if wires are exceeding 10 cm (4").

Future

Must

Should

• extend unit tests

Could

• esp32 optimization readLSBFIRST readMSBFIRST
• read(uint8_t * arr, uint8_t nr) ??
• example schema
• would it be interesting to make a fastShiftIn16() etc?
  • squeeze performance but more maintenance.?

Wont

Support

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Thank you,