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<!DOCTYPE html>
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                <section data-markdown

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                        # embedded architectures

## **Design of Autonomous Systems**
### csci 6907/4907-Section 86
### Prof. **Sibin Mohan**

---

## what is an "embedded system"?

---

## what is an "embedded system"?

- like autonomy, no _exact_ definition

---
## what is an "embedded system"?

- like autonomy, no _exact_ definition
- systems created for **specific functionality**

---

## what is an "embedded system"?

- like autonomy, no _exact_ definition
- systems created for **specific functionality**
- fixed/operational for **years**, decades even!

---

## tradeoff between
<div class="multicolumn">
<div>
<br>
<br>

**performance** vs
</div>
</div>

---

## tradeoff between

<div class="multicolumn">
<div>
<br>
<br>

**performance** vs
</div>
<div>
<ul>
    <li>power/battery life</li>
</ul>
</div>
</div>

---

## tradeoff between

<div class="multicolumn">
<div>
<br>
<br>

**performance** vs
</div>
<div>
<ul>
    <li>power/battery life</li>
    <li>less memory</li>
</ul>
</div>
</div>


---

## tradeoff between

<div class="multicolumn">
<div>
<br>
<br>

**performance** vs
</div>
<div>
<ul>
    <li>power/battery life</li>
    <li>less memory</li>
    <li>fewer peripherals</li>
</ul>
</div>
</div>


---

## tradeoff between

<div class="multicolumn">
<div>
<br>
<br>

**performance** vs
</div>
<div>
<ul>
    <li>power/battery life</li>
    <li>less memory</li>
    <li>fewer peripherals</li>
    <li>limited applications</li>
</ul>
</div>
</div>


---

## tradeoff between

<div class="multicolumn">
<div>
<br>
<br>

**performance** vs
</div>
<div>
<ul>
    <li>power/battery life</li>
    <li>less memory</li>
    <li>fewer peripherals</li>
    <li>limited applications</li>
    <li>smaller operating systems</li>
    <li><em>etc.</em></li>
</ul>
</div>
</div>

---

main reason?

## **predictability**

---

## predictability

**guarantee** the system works,

---

## predictability

**guarantee** the system works,
- correctly
- **safely**


---

## predictability

**guarantee** the system works,
- correctly
- **safely**

essentially &rarr; must be easy to **certify** the system

---

## the **wcet** problem

---

## the **wcet** problem

"worst-case execution time"

---

## the **wcet** problem

"worst-case execution time"

> **longest** execution time for a program <br>

---

## the **wcet** problem

"worst-case execution time"

> **longest** execution time for a program <br>
> on a **specific hardware platform**

---

## the **wcet** problem

"worst-case execution time"

> **longest** execution time for a program <br>
> on a **specific hardware platform**

must consider &rarr; **all possible inputs**

---

## wcet | used to ensure...

- "_schedulability_"
- resource requirements
- performance limits

of embedded and real-time programs

---

## wcet approaches

lots of approaches to computing the WCET, _e.g.,_
- [dynamic/empirical](https://www.cs.fsu.edu/~whalley/papers/tecs07.pdf) &rarr; run program lots of times  on the platform
- [static](https://www.cs.fsu.edu/~whalley/papers/tecs07.pdf) &rarr; _compile time_ analysis to compute the _worst-case paths_
- [hybrid](https://sibin.github.io/papers/2008_NCSU-Dissertation_CheckerMode_SibinMohan.pdf) &rarr; a combination of the two
- [probabilistic](https://people.ac.upc.edu/fcazorla/articles/jabella_ecrts2014_2.pdf) &rarr; combine dynamic analysis+statistical methods
- [ML-based methods](https://dl.acm.org/doi/10.1145/3570361.3615740) &rarr; applying machine-learning to the problem

---

## so what's the "problem"?

---

## so what's the "problem"?

at a high-level, the execution time distributions of applications:

<img src="./img/embedded_arch/wcet_wilhelm.png" height="550" style="display: inline-block;" title="https://www.inf.ed.ac.uk/teaching/courses/es/PDFs/lecture_11.pdf" />

Note: we see that the various methods either underestimate or overestimate the wcet. the former affects the safety, the latter wastes resources

---

### wcet analysis is a hard problem!

structural problems (in hardware/software) make it more difficult!

---

## consider this example

```c
void main()
{
    int max = 10 ;
    int sum = 0;
    for( int i = 0 ; i < max ; ++i)
        sum += i ;
}
```

---

## consider this example

```c
void main()
{
    int max = 10 ;
    int sum = 0;
    for( int i = 0 ; i < max ; ++i)
        sum += i ;
}
```

how do you compute the WCET for this code?

---

## need additional information

---

## need additional information

first, define the **processor** it runs on &rarr; say, "**P**"

---

## need additional information

first, define the **processor** it runs on &rarr; say, "**P**"
- how long each instruction takes to execute on P

---

## need additional information

first, define the **processor** it runs on &rarr; say, "**P**"
- how long each instruction takes to execute on P
- how many loop iterations?

---

## need additional information

first, define the **processor** it runs on &rarr; say, "**P**"
- how long each instruction takes to execute on P
- how many loop iterations?
- what are the startup/cleanup times for the program on P?

---

### adding in all that information, we have...

```c [1|3|4|5-6|7]
void main()         // startup cost = 100 cycles
{
int max = 15 ;  // 10 cycles
int sum = 0;    // 10 cycles
for( int i = 0 ; i < max ; ++i) // 5 cycles, once
sum += i ; // 20 cycles each iteration
}                   // cleanup cost = 120 cycles
```

---

### to calculate the wcet

```c
1   void main()         // startup cost = 100 cycles
2   {
3       int max = 15 ;  // 10 cycles
4       int sum = 0;    // 10 cycles
5       for( int i = 0 ; i < max ; ++i) // 5 cycles, once
6            sum += i ; // 20 cycles each iteration
7   }                   // cleanup cost = 120 cycles
```


$$
wcet = line_1 + line_3 + line_4 + line_5 + (line_6 * max)
$$

---

### to calculate the wcet

```c
1   void main()         // startup cost = 100 cycles
2   {
3       int max = 15 ;  // 10 cycles
4       int sum = 0;    // 10 cycles
5       for( int i = 0 ; i < max ; ++i) // 5 cycles, once
6            sum += i ; // 20 cycles each iteration
7   }                   // cleanup cost = 120 cycles
```


$$
wcet = line_1 + line_3 + line_4 + line_5 + (line_6 * max)
$$

which is fine for this simple example

---

Now consider this slight change to the above code:
```c [3]
void main( int argc, char* argv[] )
{
    int max = atoi( argv[1] ) ;     // convert the command line arg to max
    int sum = 0;
    for( int i = 0 ; i < max ; ++i) // how many iterations?
        sum += i ;
}
```

---

Now consider this slight change to the above code:
```c [3]
void main( int argc, char* argv[] )
{
    int max = atoi( argv[1] ) ;     // convert the command line arg to max
    int sum = 0;
    for( int i = 0 ; i < max ; ++i) // how many iterations?
        sum += i ;
}
```

previous equation **fails**!

---

Now consider this slight change to the above code:
```c [3]
void main( int argc, char* argv[] )
{
    int max = atoi( argv[1] ) ;     // convert the command line arg to max
    int sum = 0;
    for( int i = 0 ; i < max ; ++i) // how many iterations?
        sum += i ;
}
```

previous equation **fails**!

no longer know the value of `max` &rarr; arbitrary wcet!

---

let's make another change to the code:
```c [1-6|10-16]
#define VERY_LARGE_ARRAY+SIZE 1>>18

void main()
{
    int first_array[VERY_LARGE_ARRAY_SIZE] ;
    int second_array[VERY_LARGE_ARRAY_SIZE] ;

    int sum_first = 0;
    int sum_second = 0;
    for( int i = 0 ; i < VERY_LARGE_ARRAY_SIZE * 2 ; ++i)
    {
        if( i%2 )
            first_sum += first_array[i/2] ;
        else
            second_sum += second_array[(int)((i/2)+1)] ;
    }
}
```

---

let's make another change to the code:
```c [10-16]
#define VERY_LARGE_ARRAY+SIZE 1>>18

void main()
{
    int first_array[VERY_LARGE_ARRAY_SIZE] ;
    int second_array[VERY_LARGE_ARRAY_SIZE] ;

    int sum_first = 0;
    int sum_second = 0;
    for( int i = 0 ; i < VERY_LARGE_ARRAY_SIZE * 2 ; ++i)
    {
        if( i%2 )
            first_sum += first_array[i/2] ;
        else
            second_sum += second_array[(int)((i/2)+1)] ;
    }
}
```

can we compute wcet easily (using previous equation)?

**note:** maximum size of loop is known &rarr; `VERY_LARGE_ARRAY_SIZE * 2`

Note: pause and ask students what, if anything can go wrong?

---

### let's see what happens in each iteration

---

### let's see what happens in each iteration

| iter | operation | cache state | reason |
|--------|----------------------------|-------------------|---------------------------------------------|
| 1 | `first_array` loaded | miss | evicts whatever was previously in cache |

---

### let's see what happens in each iteration

| iter | operation | cache state | reason |
|----------------|----------------------------|-------------------|---------------------------------------------|
| 1 | `first_array` loaded | miss | evicts whatever was previously in cache |
| 2 | `second_array` loaded | miss | **evicts `first_array`** due to lack of space |

---

### let's see what happens in each iteration

| iter | operation | cache state | reason |
|----------------|----------------------------|-------------------|---------------------------------------------|
| 1 | `first_array` loaded | miss | evicts whatever was previously in cache |
| 2 | `second_array` loaded | miss | **evicts `first_array`** due to lack of space |
| 3 | `first_array` loaded again | miss | **evicts `second_array`** due to lack of space |
|...|
||

---

program will _constantly_ suffer cache misses!

---

program will _constantly_ suffer cache misses!

### loop's execution time blows up!

---

### loop's execution time blows up!

```c [11-18]
#define VERY_LARGE_ARRAY+SIZE 1>>18

void main()
{
    int first_array[VERY_LARGE_ARRAY_SIZE] ;
    int second_array[VERY_LARGE_ARRAY_SIZE] ;

    int sum_first = 0;
    int sum_second = 0;
    for( int i = 0 ; i < VERY_LARGE_ARRAY_SIZE * 2 ; ++i)
    {
        if( i%2 )
            first_sum += first_array[i/2] ;
            // 20 cycles + cache miss time (100s of cycles)
        else
            second_sum += second_array[(int)((i/2)+1)] ;
            // 20 cycles + cache miss time (100s of cycles)
    }
}
```


---

program will _constantly_ suffer cache misses!

### loop's execution time blows up!

<br>

now we need to model &rarr; **cache behavior** for program/data

---

### further complications from

other hardware features, _e.g.,_

- processor pipelining
- prefetching
- branch prediction
- multithreading
- multicore systems
- memory buses
- networks-on-chip
- and too many others to recount here...

---

processor feature &rarr; improves performance &rarr; *bad for wcet analysis*!


---

processor feature &rarr; improves performance &rarr; *bad for wcet analysis*!

### embedded/real-time systems prefer **simpler** processors

---

## embedded processors

come in myriad shapes and sizes:
1. Microcontrollers
2. Digital Signal Processors
3. Microprocessors
4. System-on-a-Chip
5. Embedded accelerators
6. ASICs and FPGAs

---

so, we focus on:

<img src="./img/stack_architecture/stack_overview.15.png" height="800">

---

## Microcontrollers


---

## Microcontrollers

from [Wikipedia](https://en.wikipedia.org/wiki/Microcontroller),
>   "A microcontroller (MC, UC, or μC) or microcontroller unit (MCU) is a small computer  <br>
> on a single integrated circuit."

---

## Microcontrollers

- most common type of "processors" in embedded systems
- **[more than 55%](https://www.embedded.com/the-two-percent-solution/)** of the world's processors!

---

## Microcontrollers

- most common type of "processors" in embedded systems
- **[more than 55%](https://www.embedded.com/the-two-percent-solution/)** of the world's processors!
- **small, yet critical**, systems

---

## Microcontrollers

- most common type of "processors" in embedded systems
- **[more than 55%](https://www.embedded.com/the-two-percent-solution/)** of the world's processors!
- **small, yet critical**, systems
    - car engine control
    - implantable medical devices
    - thermal monitoring
    - millions (billions?) of other applications!

---

## microcontroller hardware features

---

## microcontroller hardware features

| component | details |
|-----------|---------|
| one CPU core | typically simple `4` or `8` bit chips |

Note:
Processor bit width refers to the amount of data (in bits) a CPU can process in a single operation, determining its data handling capacity and memory addressing ability, with 32-bit and 64-bit being common standards, where 64-bit CPUs handle larger chunks of data (8 bytes) and access vastly more RAM than 32-bit ones (4 bytes)

---

## microcontroller hardware features

| component | details |
|-----------|---------|
| one CPU core | typically simple `4` or `8` bit chips |
| small pipelined architectures | sometimes `2` or `4` stage pipelines |

---

## microcontroller hardware features

| component | details |
|-----------|---------|
| one CPU core | typically simple `4` or `8` bit chips |
| small pipelined architectures | sometimes `2` or `4` stage pipelines |
| some limited memory | typically a few hundred kilobytes, perhaps in the form of EEPROMs or FLASH |

---

## microcontroller hardware features

| component | details |
|-----------|---------|
| one CPU core | typically simple `4` or `8` bit chips |
| small pipelined architectures | sometimes `2` or `4` stage pipelines |
| some limited memory | typically a few hundred kilobytes, perhaps in the form of EEPROMs or FLASH |
| programmable I/O | to interact with the real world |

---

## microcontroller hardware features [contd.]

| component | details |
|-----------|---------|
| low operating frequencies | e.g., `4 KHz`; simpler/older processors, yet more predictable |

Note:
processor "frequency is the rate at which a CPU executes basic operations, measured in gigahertz (GHz), indicating billions of cycles per second, essentially how many "ticks" the processor makes to process instructions;

---

## microcontroller hardware features [contd.]

| component | details |
|-----------|---------|
| low operating frequencies | e.g., `4 KHz`; simpler/older processors, yet more predictable |
| low power consumption | in the **milliwatts** or **microwatts** ranges; might even be **nanowatts** when the system is _sleeping_ |

---

## microcontroller hardware features [contd.]

| component | details |
|-----------|---------|
| low operating frequencies | e.g., `4 KHz`; simpler/older processors, yet more predictable |
| low power consumption | in the **milliwatts** or **microwatts** ranges; might even be **nanowatts** when the system is _sleeping_ |
| interrupts (some programmable) | often _real-time_ (fixed/low latency) |

Note:
this is the **crux** of how we can implement "real-time" behavior. Low latency/deterministic interrupts

---

## microcontroller hardware features [contd.]

| component | details |
|-----------|---------|
| low operating frequencies | e.g., `4 KHz`; simpler/older processors, yet more predictable |
| low power consumption | in the **milliwatts** or **microwatts** ranges; might even be **nanowatts** when the system is _sleeping_ |
| interrupts (some programmable) | often _real-time_ (fixed/low latency) |
| general-purpose I/O (GPIO) pins | for I/O |

---

## microcontroller hardware features [contd.]

| component | details |
|-----------|---------|
| low operating frequencies | e.g., `4 KHz`; simpler/older processors, yet more predictable |
| low power consumption | in the **milliwatts** or **microwatts** ranges; might even be **nanowatts** when the system is _sleeping_ |
| interrupts (some programmable) | often _real-time_ (fixed/low latency) |
| several general-purpose I/O (GPIO) pins | for I/O |
| timers | e.g., a programmable interval timer (PIT) |
||

---

### **Additional features** found on some microcontrollers:

---

### **Additional features** found on some microcontrollers:

| component | details |
|-----------|---------|
| analog to digital (ADC) convertors | to convert incoming (real-world, sensor) data to a digital form that the uC can operate on |

Note:
- because the real world is "analog"/continuous

---

### **Additional features** found on some microcontrollers:

| component | details |
|-----------|---------|
| analog to digital (ADC) convertors | to convert incoming (real-world, sensor) data to a digital form that the uC can operate on |
| digital-to-analog (DAC) convertor | to do the opposite, convert from digital to analog signals to send outputs in that form |

Note:
- because the real world is "analog"/continuous

---

### **Additional features** found on some microcontrollers:

| component | details |
|-----------|---------|
| analog to digital (ADC) convertors | to convert incoming (real-world, sensor) data to a digital form that the uC can operate on |
| digital-to-analog (DAC) convertor | to do the opposite, convert from digital to analog signals to send outputs in that form |
| universal async tx/rx (UART) | to receive/send data over a _serial_ line |

---

### **Additional features** [contd.]:

| component | details |
|-----------|---------|
| pulse width modulation (PWM) | so that the CPU can control **motors** (significant for us in autonomous/automotive systems), power systems, resistive loads, etc. |

---

### **Additional features** found on some microcontrollers:

| component | details |
|-----------|---------|
| pulse width modulation (PWM) | so that the CPU can control **motors** (significant for us in autonomous/automotive systems), power systems, resistive loads, etc. |
| JTAG interface | debugging interface |
||

---

### Microcontroller programs and data

---

### Microcontroller programs and data

- are small &rarr; must fit in (limited) memory

---

### Microcontroller programs and data

- are small &rarr; must fit in (limited) memory
- often directly programmed in **assembly**!

---

### Microcontroller programs and data

- are small &rarr; must fit in (limited) memory
- often directly programmed in **assembly**!
    - assembly code might need _hand tuning_

Note: hand-tuning is for both, performance as well as fitting into the limited memory

---

### Microcontroller programs and data

- are small &rarr; must fit in (limited) memory
- often directly programmed in **assembly**!
    - assembly code might need _hand tuning_
- **C** is another popular language

---

### Microcontroller programs and data

- are small &rarr; must fit in (limited) memory
- often directly programmed in **assembly**!
    - assembly code might need _hand tuning_
- **C** is another popular language
- **no operating systems** (or very rare)!

Note:
- "**bare metal execution"

---

### Microcontroller programs and data

- are small &rarr; must fit in (limited) memory
- often directly programmed in **assembly**!
    - assembly code might need _hand tuning_
- **C** is another popular language
- **no operating systems** (or very rare)!
- sometimes use special-purpose programming languages/instructions


---

### popular microcontroller families:

|||||
|----|----|----|----|
|<center><img src="./img/embedded_arch/ATmega169-MLF.jpg" height="100"><br>Atmel ATmega</center> | <center><img src="./img/embedded_arch/Microchip_PIC24HJ32GP202.jpg" height="100"><br>Microchip Technology</center> | <center><img src="./img/embedded_arch/Motorola_68HC11.jpg" height="100"><br>Motorola (Freescale)<br></center> | <center><img src="./img/embedded_arch/NXP_LPC2387FBD100-5543.jpg" height="100"><br>NXP</center> |
||

---

## digital signal processor (dsp)

---

## what is a dsp?

- specialized microcontrollers optimized for **digital signal processing**

---

## what is a dsp?

- specialized microcontrollers optimized for **digital signal processing**

<img src="./img/embedded_arch/dsp_example.avif" height="500" title="dsp signal example">

---

## what is a dsp?

- specialized microcontrollers optimized for digital signal processing
- widely used in:
    - audio processing
    - radar and sonar
    - speech recognition systems
    - image processing
    - satellites
    - telecommunications
    - mobile phones
    - televisions

---

## main goals of dsp

- **isolate**, **measure**, **compress**, filter analog signals in the real world
- often have **stringent real-time constraints**

---

<!-- .slide: data-background="white" -->

## example

- texas instruments dsp chip, [tms320 series](https://www.ti.com/lit/ug/spruh79c/spruh79c.pdf?ts=1736945981001)
- one of the most famous examples of dsp systems

<br>
<img src="./img/embedded_arch/TI_DSP.jpg" height="500" title="Texas Instruments DSP Chip">

---

## typical digital signal processing

- **repetitive mathematical operations**
    - over a large number of samples
    - in real-time

---

## typical digital signal processing

- **repetitive mathematical operations**
    - over a large number of samples
    - in real-time

<br>
<br>


### "**multiply-accumulate**" (mac) algorithm

Note:
Imagine you are reading a sensor that measures temperature every second. The signal is "noisy" (it jumps around erratically). To smooth it, you don't just look at the current number; you look at the average of the current number and the previous two numbers.

This requires two repetitive steps that are the heart of DSP:

Multiplication: Weighting the data points.

Accumulation: Adding them together.

---

### "**multiply-accumulate**" (mac) algorithm

```python [1-3|4-6|7-10|11-14]
signal = [10, 22, 10, 12]
weights = [0.5, 0.25, 0.25]
output = []

# This loop is the "repetitive" nature of DSP
for i in range(2, len(signal)):
    # The MAC (Multiply-Accumulate) Operation
    val = (signal[i]   * weights[0]) + \