Hardware Communication

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# 181U Spring 2020
### Hardware Communication

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.footer[
- Geoffrey Brown, 2020
- 181U
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# Agenda

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* Serial
  - Examples
* SPI
  - Examples
* I2C
  - Examples
* Analog

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# Serial Protocol
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![](images/serial-topology.png# w-40pct fr)

* Oldest protocol but still widely used.  
* Depends upon local clocks to decode digital waveform
* Used for
    - UART (in lab, the serial interface is through a UART)
    - GPS receivers
    - Cell phone modules

![](images/space.png# w-20pct)
![](images/serial-frame.png# w-60pct)

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# Typical Configuration of Serial Device
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![](images/space.png# w-30pct)
![](images/uart-wiring.png# w-40pct)

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# Decoding Serial Protocol

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![](images/serial-decoding.png# w-50pct fr)

* Sender and Receiver each have local clocks that are within ~2%
* Sender and Receiver programmed to use the same "baud rate"  (9600-115200 baud)
* Sender transmits a frame, making transitions at bit boundaries
    - frame has start bit (0), 8 bits, at least one stop bit (1)
* Receiver samples the input signal looking for a start bit
    - after seeing start bit, skip 1.5 baud times to bit 0
    - then skip 1 baud time for each bit

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# Serial Driver Software

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* Serial (UART) hardware typically has
    - a send buffer
    - a receive buffer
    - a status register
    - a configuration register
* Driver
    - configures baud rate and protocol (configuration register)
    - sending
       - write one byte to send buffer
       - wait for byte to be transmitted (check status register)
    - receiving
       - wait for byte to be received
       - read byte from receive buffer
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# Serial Drivers (cont.)

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* Two types of drivers
    - Polling  -- busy waiting.  This is **very** inefficient
    - Interrupt driven -- interrupt handler keeps uart busy from queues fed/emptied by OS

![](images/space.png# w-20pct)
![](images/interrupt-queues.png# w-60pct)

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# UART Interrupt Handler

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```C
void USART1_IRQHandler(void)
{
  if(USART_GetITStatus(USART1, USART_IT_RXNE) != RESET)
    {
      uint8_t  data;

// buffer the data (or toss it if there's no room 
      // Flow control will prevent this

data = USART_ReceiveData(USART1) & 0xff;
      if (!Enqueue(&UART1_RXq, data))
        RxOverflow = 1;
    }
  ```
  <br>

```C
  if(USART_GetITStatus(USART1, USART_IT_TXE) != RESET)
    {   
      uint8_t data;

/* Write one byte to the transmit data register */

if (Dequeue(&UART1_TXq, &data)){
          USART_SendData(USART1, data);
      } else {
          // if we have nothing to send, 
          // disable the interrupt
          // and wait for a kick

USART_ITConfig(USART1, USART_IT_TXE, DISABLE);
          TxPrimed = 0;
      }
    }
}
```

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# Interrupt Driven UART User Code

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![](images/circular-queue.png# w-50pct fr)

```C
int getchar(void)
{
  uint8_t data;
  while (!Dequeue(&UART1_RXq, &data));
  return data;
}

int putchar(int c)
{
   while (!Enqueue(&UART1_TXq, c));
   if (!TxPrimed) {
     TxPrimed = 1;
     USART_ITConfig(USART1, USART_IT_TXE, ENABLE);
   }
}
```

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# SPI

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![](images/spi-topology.png# w-40pct fr)

* SPI is a point-to-point *synchronous* protocol with a single *master* and one or more *slaves*
* In contrast, the serial protocol is point-to-point *asynchronous* (but with a specified time base)
* Signals:
   - SCK : clock from master to slave
   - MOSI : data from master to slave ("Master Out Slave In")
   - MISO : data from slave to master ("Slave Out Master In")
   - SS : slave select (one for each slave)

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# SPI Logical Architecture

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![](images/space.png# w-20pct)
![](images/spi-logical.png# w-60pct)

* On every clock event (typically 0->1 transition), the Master and Slave registers are "shifted"

`master[0],master[1:7],slave[0],slave[1:7] = slave[7],master[0:6],master[7],slave[0:6]`

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# SPI Protocol

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![](images/space.png# w-30pct)
![](images/spi-logical.png# w-40pct)

![](images/space.png# w-20pct)
![](images/spi-timing.png# w-60pct)

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# SPI Example -- serial EEPROM (memory)
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![](images/eeprom-pinout.png# w-20pct)
![](images/space.png# 40-pct)
![](images/eeprom-read-status.png# w-30pct)

* SPI protocol implements byte (or word) transfer
* Every device has a message protocol implemented on top of this
* For example, with serial eproms, the master sends a command followed by 0xFF and the slave responds with a value

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# SPI Example -- color LCD display
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![](images/space.png# w-30pct)
![](images/lcd-model.png# w-40pct)

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# SPI Example -- SD Card
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![](images/space.png# w-30pct)
![](images/sd-card.png# w-40pct)

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# SPI Example -- SD Card Transactions
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![](images/sd-transactions.png# w-50pct fr)

* CMD0 :  Reset the SD card 
* CMD1 : Initialize the card 
* CMD13 : Request card status 
* CMD16 : Set block length 
* CMD17 : Read single block 
* CMD24 : Write single block

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# SPI Example -- Display/SD card reader
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![](images/space.png# w-30pct)
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# I2C -- 2 wire broadcast bus
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![](images/space.png# w-20pct)
![](images/I2C-configuration.png# w-60pct)

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# I2C -- transactions
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![](images/space.png# w-20pct)
![](images/i2c-transactions.png# w-60pct)

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# I2C Example -- Wii Nunchuck
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![](images/nunchuck-drawing.png# w-40pct)
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# Analog Signals (digital <-> voltage)
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* Analog to Digital Converter (ADC) : samples analog signal an produces digital representation
* Digital to Analog Converter (DAC) : converts digital representation into analog voltage
* These may be integral to processor (e.g. STM32L476) or external devices
* Frequently, the conversion must be "triggered" at regular intervals (for example sampling a continuous signal or
converting an audio file to an analog signal).
* Example architecture of a DAC channel:

![](images/space.png# w-30pct)
![](images/dac-channel.png# w-40pct)

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# Analog Conversion -- Successive approximation
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![](images/successive-approx.png# w-50pct)
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# Analog to Digital Converter Example
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* STM32 ADC is complicated
   - multiple input sources including internal references
   - mulitple trigger conditions
   - complex programming

![](images/space.png# w-30pct)
![](images/adc-architecture.png# w-40pct)

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# Summary
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* Common hardware communication protocols
   - Serial  (asynchronous point-to-point)
   - SPI     (synchronous point-to-point)
   - I2C     (synchronous master/slave bus)