add daisy chain example

examples/74hc595_daisy_chain/74hc595_daisy_chain.md

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+# About
+
+The example here shows the wiring and codes for daisy-chained shift registers.
+
+The following items are used:
+- Two Shift Registers: [74HC595](https://www.adafruit.com/product/450)
+- One Level Shifter: [74AHCT125](https://www.adafruit.com/product/1787)
+- One LED Display: [10 Segment Light Bar LED](https://www.adafruit.com/product/1815)
+- One microcontroller board: [ItsyBitsy M0 Express](https://www.adafruit.com/product/3727)
+
+In addition, some other items are needed as well:
+- Through-hole 2K Resistor: [CFR-25JR-52-2K](https://www.digikey.com/en/products/detail/yageo/CFR-25JR-52-2K/11981)
+- Jumper wires
+
+The idea is to let the microcontroller to communicate with the shift registers through SPI, and then control 10 LEDs.
+The level shifter **74AHCT125** helps to translate 3.3V signal to 5.0V, since this example is trying to use 5.0V to drive LEDs and chips.
+If one uses 3.3V for all LEDs and chips, the level shifter is not going to be needed.
+
+For the daisy chain connection, this guide, [SPI interface Tutorial](https://www.best-microcontroller-projects.com/spi-interface.html), can provide some basic ideas.
+The guide, [Serial to Parallel Shifting-Out with a 74HC595](https://www.arduino.cc/en/Tutorial/Foundations/ShiftOut), can also provide some connection details for the 74HC595 chip.
+
+# Wiring
+
+With the board `Itsy Bitsy M0 Express`, pins can be connected as follows:
+
+![Itsy Bitsy M0 Express with Two Shift Registers, LEDs, and Level Shifter Fritzing](figs/ItsyBitsyM0_w_LEDs_and_two_shift_registers_bb.png)
+
+In the wiring, the ItsyBitsy M0 is not powered, given that one can power it up through USB cable when programming it.
+
+Also, one may notice that, for the first 8 LEDs (from left to right), the pins from the 74HC595 at the *end* of the daisy chain are fully utilized, which means that the end of the daisy chain is actually the first device, the index 0 device.
+
+Therefore,
+- for pin index 0~7, the value will be sent to the device at the end of the daisy chain
+- for pin index 8~15, the value will be sent to the device the second end of the daisy chain (which is the head of the daisy chain in this example)
+
+# Coding
+
+The example code can be found in `74hc595_daisy_chain.py`.
+Here are some notes about the codes:
+
+## Import Modules
+
+```python
+import time
+import board
+import digitalio
+import adafruit_74hc595
+```
+
+## Instantiate Class
+
+One need to provide the number of shift registers when instantiating the class.
+
+```python
+# note: D2 port is close to SCK and MOSI pins for Itsy Bitsy M0
+latch_pin = digitalio.DigitalInOut(board.D2)
+sr = adafruit_74hc595.ShiftRegister74HC595(board.SPI(), latch_pin, number_of_shift_registers=2)
+
+shift_register_pin_num = 10
+shift_register_pins = [sr.get_pin(n) for n in range(shift_register_pin_num)]
+```
+
+## Do Something with the Pins
+
+One simple example is to blink all LEDs together:
+
+```python
+while True:
+    # turn all LEDs on
+    for i in range(shift_register_pin_num):
+        shift_register_pins[i].value = True
+    time.sleep(0.5)
+
+    # turn all LEDs off
+    for i in range(shift_register_pin_num):
+        shift_register_pins[i].value = False
+    time.sleep(0.5)
+```
+
+A more complicated example is to turn on/off each LEDs one-by-one:
+```python
+bar_ind_current = 0 # current index
+shift_register_pins[bar_ind_current].value = True
+
+while True:
+    # iterate through every LED
+    bar_ind_last = bar_ind_current
+    bar_ind_current = bar_ind_last + 1
+    if bar_ind_current <= (shift_register_pin_num-1):
+        shift_register_pins[bar_ind_last].value = False
+        shift_register_pins[bar_ind_current].value = True
+    else:
+        bar_ind_current = 0
+        shift_register_pins[bar_ind_last].value = False
+        shift_register_pins[bar_ind_current].value = True
+    time.sleep(0.1)
+```
+
+Here is the result one can get:
+
+![Itsy Bitsy M0 Express with Two Shift Registers, LEDs, and Level Shifter Demo](figs/ItsyBitsyM0_w_LEDs_and_two_shift_registers.gif)

examples/74hc595_daisy_chain/74hc595_daisy_chain.md.license

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+SPDX-FileCopyrightText: 2021 Zichao Hou
+
+SPDX-License-Identifier: MIT

examples/74hc595_daisy_chain/74hc595_daisy_chain.py

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+# SPDX-FileCopyrightText: 2021 Zichao Hou
+# SPDX-License-Identifier: MIT
+
+import time
+import board
+import digitalio
+import adafruit_74hc595
+
+# note: D2 port is close to SCK and MOSI pins for Itsy Bitsy M0
+latch_pin = digitalio.DigitalInOut(board.D2)
+sr = adafruit_74hc595.ShiftRegister74HC595(
+    board.SPI(), latch_pin, number_of_shift_registers=2
+)
+
+shift_register_pin_num = 10
+shift_register_pins = [sr.get_pin(n) for n in range(shift_register_pin_num)]
+
+# turn all pins off
+for pin in shift_register_pins:
+    pin.value = False
+
+bar_ind_current = 0  # current index
+shift_register_pins[bar_ind_current].value = True
+
+while True:
+    # iterate through every LED
+    bar_ind_last = bar_ind_current
+    bar_ind_current = bar_ind_last + 1
+    if bar_ind_current <= (shift_register_pin_num - 1):
+        shift_register_pins[bar_ind_last].value = False
+        shift_register_pins[bar_ind_current].value = True
+    else:
+        bar_ind_current = 0
+        shift_register_pins[bar_ind_last].value = False
+        shift_register_pins[bar_ind_current].value = True
+    time.sleep(0.1)

examples/74hc595_daisy_chain/figs/ItsyBitsyM0_w_LEDs_and_two_shift_registers.gif.license

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+SPDX-FileCopyrightText: 2021 Zichao Hou
+
+SPDX-License-Identifier: MIT

examples/74hc595_daisy_chain/figs/ItsyBitsyM0_w_LEDs_and_two_shift_registers_bb.png.license

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+SPDX-FileCopyrightText: 2021 Zichao Hou
+
+SPDX-License-Identifier: MIT