Micro_Controllers - RicoJia/notes GitHub Wiki

• one shot timer is: when you push a button once, an LED will keep on for a while, then it will stay off again.

  • E.g, you press a button and you get a stable on waveform for a while.

  

  one_shot_waveform
  • pinouts of 555 timer: Discharge is key to having this to work

  

  555 Timer Pinouts

  • Internal Circuit of 555

    • Set output pin to high:

    

    Set 555
    • Set output pin to low, because we need to feed high to R thru C2. We want 6 to be Vcc.
      • Note that we assume we have released the button before we want to feed high to R.

    

    Reset 555
    • So we can put a cap, which takes time to charge and discharge, thus set 555 to high and low

    

    With capacitor
    • Finally we can drive an LED

    

    555_full

• Auto-reload timer:

  • in STM32, there's auto-reload timer, whose value will reset the timer back to 0. (ARR)
    • the timer might be a count-up timer or count-down timer

• clock stretching

Basics

• 255 devices, 2^7.
• aka TWI communication, Same logical order as serial pins, PWM pins, SPI pins, Digital Write pins, and external interrupts
• Slow compared to USB, PCI, but good for EEPROM (e-e-prom) chips -Principle
• synchronous communication, means clocked, (SCL)
  • Needs pull-up resistor for high/low switching
• No chip select, because address is being sent thru data (SDA)
  • 8 bit address
  • each device has a unique address. That address should be settable.
  • protocols
    • Write protocol

      start bit 
      8 bit - address (only the right device will keep listening, also, the first bit tells read/write)
      8 bit - command
      8 bit - value
      stop bit
      

    • Read Protocol

      start bit
      8 bit address
      8 bit command (something we want to change)
      restart bit 
      8 bit address
      RECV (receive)
      AK
      repeat...
      stop bit
      

      • You want to read all the bytes from the buffer, otherwise it's not a complete transaction
• DMA (Direct Memory Access)
  • This allows I2C to directly write to the registers, instead of using the processor
  • Good for slow processes, because will offload the processor even more.
• Downsides:
  • if mcu freezes, the device doesn't know this and will hang. You need to restart the power.
• Polling vs Interrupts:
  • Interrupt is hardware line that tells the CPU
  • Polling is a software steady check
  • Interrupt pros:
    • Interrupt is much better than polling, cuz polling might have data loss
    • good for infrequent uses.
  • Polling pros:
    • synchronous (i.e, you know when the poll happens)
    • frequent but not urgent
• clock-stretching

  • If the slave is too slow, it can hold the clock to ground level, to pause the I2C

    

  • without clock_stretching, in the above pic, the master will read a NACK (not acknowledged) from SDA

I2C in action

1. Two devices sharing the same I2C bus using interrupts: how does that work?

• “the slave device sets that GPIO to high, which raises an interrupt with the master device. The master device then reads a “interrupt source” register which indicates why the slave device raised the interrupt, and goes on to query the device for the information.”

Libraries:

• Wire Library: Arduino, ESP8266, STM32
• TinyWire Library: Attiny