Go on very small hardware (Part 2)
============================================================


 [![STM32F030F4P6](https://ziutek.github.io/images/mcu/f030-demo-board/board.jpg)][1] 

At the end of the [first part][2] of this article I promised to write something about  _interfaces_ . I don’t want to write here a complete or even brief lecture about the interfaces. Instead, I’ll show a simple example how to define and use an interface, and then, how to take advantage of ubiquitous  _io.Writer_  interface. There will also be a few words about  _reflection_  and  _semihosting_ .

Interfaces are a crucial part of Go language. If you want to learn more about them, I suggest to read [Effective Go][3] and [Russ Cox article][4].

### Concurrent Blinky – revisited

When you read the code of previous examples you probably noticed a counterintuitive way to turn the LED on or off. The  _Set_  method was used to turn the LED off and the  _Clear_  method was used to turn the LED on. This is due to driving the LEDs in open-drain configuration. What we can do to make the code less confusing? Let’s define the  _LED_  type with  _On_  and  _Off_  methods:

```
type LED struct {
	pin gpio.Pin
}

func (led LED) On() {
	led.pin.Clear()
}

func (led LED) Off() {
	led.pin.Set()
}

```

Now we can simply call `led.On()` and `led.Off()` which no longer raises any doubts.

In all previous examples I tried to use the same open-drain configuration to don’t complicate the code. But in the last example, it would be easier for me to connect the third LED between GND and PA3 pins and configure PA3 in push-pull mode. The next example will use a LED connected this way.

But our new  _LED_  type doesn’t support the push-pull configuration. In fact, we should call it  _OpenDrainLED_  and define another  _PushPullLED_  type:

```
type PushPullLED struct {
	pin gpio.Pin
}

func (led PushPullLED) On() {
	led.pin.Set()
}

func (led PushPullLED) Off() {
	led.pin.Clear()
}

```

Note, that both types have the same methods that work the same. It would be nice if the code that operates on LEDs could use both types, without paying attention to which one it uses at the moment. The  _interface type_  comes to help:

```
package main

import (
	"delay"

	"stm32/hal/gpio"
	"stm32/hal/system"
	"stm32/hal/system/timer/systick"
)

type LED interface {
	On()
	Off()
}

type PushPullLED struct{ pin gpio.Pin }

func (led PushPullLED) On()  {
	led.pin.Set()
}

func (led PushPullLED) Off() {
	led.pin.Clear()
}

func MakePushPullLED(pin gpio.Pin) PushPullLED {
	pin.Setup(&gpio.Config{Mode: gpio.Out, Driver: gpio.PushPull})
	return PushPullLED{pin}
}

type OpenDrainLED struct{ pin gpio.Pin }

func (led OpenDrainLED) On()  {
	led.pin.Clear()
}

func (led OpenDrainLED) Off() {
	led.pin.Set()
}

func MakeOpenDrainLED(pin gpio.Pin) OpenDrainLED {
	pin.Setup(&gpio.Config{Mode: gpio.Out, Driver: gpio.OpenDrain})
	return OpenDrainLED{pin}
}

var led1, led2 LED

func init() {
	system.SetupPLL(8, 1, 48/8)
	systick.Setup(2e6)

	gpio.A.EnableClock(false)
	led1 = MakeOpenDrainLED(gpio.A.Pin(4))
	led2 = MakePushPullLED(gpio.A.Pin(3))
}

func blinky(led LED, period int) {
	for {
		led.On()
		delay.Millisec(100)
		led.Off()
		delay.Millisec(period - 100)
	}
}

func main() {
	go blinky(led1, 500)
	blinky(led2, 1000)
}

```

We’ve defined  _LED_  interface that has two methods:  _On_  and  _Off_ . The  _PushPullLED_  and  _OpenDrainLED_ types represent two ways of driving LEDs. We also defined two  _Make_  _*LED_  functions which act as constructors. Both types implement the  _LED_  interface, so the values of these types can be assigned to the variables of type  _LED_ :

```
led1 = MakeOpenDrainLED(gpio.A.Pin(4))
led2 = MakePushPullLED(gpio.A.Pin(3))

```

In this case the assignability is checked at compile time. After the assignment the  _led1_  variable contains `OpenDrainLED{gpio.A.Pin(4)}` and a pointer to the method set of the  _OpenDrainLED_  type. The `led1.On()` call roughly corresponds to the following C code:

```
led1.methods->On(led1.value)

```

As you can see, this is quite inexpensive abstraction if only consider the function call overhead.

But any assigment to an interface causes to include a lot of information about the assigned type. There can be a lot information in case of complex type which consists of many other types:

```
$ egc
$ arm-none-eabi-size cortexm0.elf 
   text    data     bss     dec     hex filename
  10356     196     212   10764    2a0c cortexm0.elf

```

If we don’t use [reflection][5] we can save some bytes by avoid to include the names of types and struct fields:

```
$ egc -nf -nt
$ arm-none-eabi-size cortexm0.elf 
   text    data     bss     dec     hex filename
  10312     196     212   10720    29e0 cortexm0.elf

```

The resulted binary still contains some necessary information about types and full information about all exported methods (with names). This information is need for checking assignability at runtime, mainly when you assign one value stored in the interface variable to any other variable.

We can also remove type and field names from imported packages by recompiling them all:

```
$ cd $HOME/emgo
$ ./clean.sh
$ cd $HOME/firstemgo
$ egc -nf -nt
$ arm-none-eabi-size cortexm0.elf 
   text    data     bss     dec     hex filename
  10272     196     212   10680    29b8 cortexm0.elf

```

Let’s load this program to see does it work as expected. This time we’ll use the [st-flash][6] command:

```
$ arm-none-eabi-objcopy -O binary cortexm0.elf cortexm0.bin
$ st-flash write cortexm0.bin 0x8000000
st-flash 1.4.0-33-gd76e3c7
2018-04-10T22:04:34 INFO usb.c: -- exit_dfu_mode
2018-04-10T22:04:34 INFO common.c: Loading device parameters....
2018-04-10T22:04:34 INFO common.c: Device connected is: F0 small device, id 0x10006444
2018-04-10T22:04:34 INFO common.c: SRAM size: 0x1000 bytes (4 KiB), Flash: 0x4000 bytes (16 KiB) in pages of 1024 bytes
2018-04-10T22:04:34 INFO common.c: Attempting to write 10468 (0x28e4) bytes to stm32 address: 134217728 (0x8000000)
Flash page at addr: 0x08002800 erased
2018-04-10T22:04:34 INFO common.c: Finished erasing 11 pages of 1024 (0x400) bytes
2018-04-10T22:04:34 INFO common.c: Starting Flash write for VL/F0/F3/F1_XL core id
2018-04-10T22:04:34 INFO flash_loader.c: Successfully loaded flash loader in sram
 11/11 pages written
2018-04-10T22:04:35 INFO common.c: Starting verification of write complete
2018-04-10T22:04:35 INFO common.c: Flash written and verified! jolly good!

```

I didn’t connected the NRST signal to the programmer so the  _—reset_  option can’t be used and the reset button have to be pressed to run the program.

![Interfaces](https://ziutek.github.io/images/mcu/f030-demo-board/interfaces.png)

It seems that the  _st-flash_  works a bit unreliably with this board (often requires reseting the ST-LINK dongle). Additionally, the current version doesn’t issue the reset command over SWD (uses only NRST signal). The software reset isn’t realiable however it usually works and lack of it introduces inconvenience. For this board-programmer pair the  _OpenOCD_  works much better.

### UART

UART (Universal Aynchronous Receiver-Transmitter) is still one of the most important peripherals of today’s microcontrollers. Its advantage is unique combination of the following properties:

*   relatively high speed,

*   only two signal lines (even one in case of half-duplex communication),

*   symmetry of roles,

*   synchronous in-band signaling about new data (start bit),

*   accurate timing inside transmitted word.

This causes that UART, originally intedned to transmit asynchronous messages consisting of 7-9 bit words, is also used to efficiently implement various other phisical protocols such as used by [WS28xx LEDs][7] or [1-wire][8] devices.

However, we will use the UART in its usual role: to printing text messages from our program.

```
package main

import (
	"io"
	"rtos"

	"stm32/hal/dma"
	"stm32/hal/gpio"
	"stm32/hal/irq"
	"stm32/hal/system"
	"stm32/hal/system/timer/systick"
	"stm32/hal/usart"
)

var tts *usart.Driver

func init() {
	system.SetupPLL(8, 1, 48/8)
	systick.Setup(2e6)

	gpio.A.EnableClock(true)
	tx := gpio.A.Pin(9)

	tx.Setup(&gpio.Config{Mode: gpio.Alt})
	tx.SetAltFunc(gpio.USART1_AF1)
	d := dma.DMA1
	d.EnableClock(true)
	tts = usart.NewDriver(usart.USART1, d.Channel(2, 0), nil, nil)
	tts.Periph().EnableClock(true)
	tts.Periph().SetBaudRate(115200)
	tts.Periph().Enable()
	tts.EnableTx()

	rtos.IRQ(irq.USART1).Enable()
	rtos.IRQ(irq.DMA1_Channel2_3).Enable()
}

func main() {
	io.WriteString(tts, "Hello, World!\r\n")
}

func ttsISR() {
	tts.ISR()
}

func ttsDMAISR() {
	tts.TxDMAISR()
}

//c:__attribute__((section(".ISRs")))
var ISRs = [...]func(){
	irq.USART1:          ttsISR,
	irq.DMA1_Channel2_3: ttsDMAISR,
}

```

You can find this code slightly complicated but for now there is no simpler UART driver in STM32 HAL (simple polling driver will be probably useful in some cases). The  _usart.Driver_  is efficient driver that uses DMA and interrupts to ofload the CPU.

STM32 USART peripheral provides traditional UART and its synchronous version. To use it as output we have to connect its Tx signal to the right GPIO pin:

```
tx.Setup(&gpio.Config{Mode: gpio.Alt})
tx.SetAltFunc(gpio.USART1_AF1)

```

The  _usart.Driver_  is configured in Tx-only mode (rxdma and rxbuf are set to nil):

```
tts = usart.NewDriver(usart.USART1, d.Channel(2, 0), nil, nil)

```

We use its  _WriteString_  method to print the famous sentence. Let’s clean everything and compile this program:

```
$ cd $HOME/emgo
$ ./clean.sh
$ cd $HOME/firstemgo
$ egc
$ arm-none-eabi-size cortexm0.elf
  text	   data	    bss	    dec	    hex	filename
  12728	    236	    176	  13140	   3354	cortexm0.elf

```

To see something you need an UART peripheral in your PC.

**Do not use RS232 port or USB to RS232 converter!**

The STM32 family uses 3.3 V logic but RS232 can produce from -15 V to +15 V which will probably demage your MCU. You need USB to UART converter that uses 3.3 V logic. Popular converters are based on FT232 or CP2102 chips.

![UART](https://ziutek.github.io/images/mcu/f030-demo-board/uart.jpg)

You also need some terminal emulator program (I prefer [picocom][9]). Flash the new image, run the terminal emulator and press the reset button a few times:

```
$ openocd -d0 -f interface/stlink.cfg -f target/stm32f0x.cfg -c 'init; program cortexm0.elf; reset run; exit'
Open On-Chip Debugger 0.10.0+dev-00319-g8f1f912a (2018-03-07-19:20)
Licensed under GNU GPL v2
For bug reports, read
        http://openocd.org/doc/doxygen/bugs.html
debug_level: 0
adapter speed: 1000 kHz
adapter_nsrst_delay: 100
none separate
adapter speed: 950 kHz
target halted due to debug-request, current mode: Thread 
xPSR: 0xc1000000 pc: 0x080016f4 msp: 0x20000a20
adapter speed: 4000 kHz
** Programming Started **
auto erase enabled
target halted due to breakpoint, current mode: Thread 
xPSR: 0x61000000 pc: 0x2000003a msp: 0x20000a20
wrote 13312 bytes from file cortexm0.elf in 1.020185s (12.743 KiB/s)
** Programming Finished **
adapter speed: 950 kHz
$
$ picocom -b 115200 /dev/ttyUSB0 
picocom v3.1

port is        : /dev/ttyUSB0
flowcontrol    : none
baudrate is    : 115200
parity is      : none
databits are   : 8
stopbits are   : 1
escape is      : C-a
local echo is  : no
noinit is      : no
noreset is     : no
hangup is      : no
nolock is      : no
send_cmd is    : sz -vv
receive_cmd is : rz -vv -E
imap is        : 
omap is        : 
emap is        : crcrlf,delbs,
logfile is     : none
initstring     : none
exit_after is  : not set
exit is        : no

Type [C-a] [C-h] to see available commands
Terminal ready
Hello, World!
Hello, World!
Hello, World!

```

Every press of the reset button produces new “Hello, World!” line. Everything works as expected.

To see bi-directional UART code for this MCU check out [this example][10].

### io.Writer

The  _io.Writer_  interface is probably the second most commonly used interface type in Go, right after the  _error_  interface. Its definition looks like this:

```
type Writer interface {
	Write(p []byte) (n int, err error)
}

```

 _usart.Driver_  implements  _io.Writer_  so we can replace:

```
tts.WriteString("Hello, World!\r\n")

```

with

```
io.WriteString(tts, "Hello, World!\r\n")

```

Additionally you need to add the  _io_  package to the  _import_  section.

The declaration of  _io.WriteString_  function looks as follows:

```
func WriteString(w Writer, s string) (n int, err error)

```

As you can see, the  _io.WriteString_  allows to write strings using any type that implements  _io.Writer_ interface. Internally it check does the underlying type has  _WriteString_  method and uses it instead of  _Write_  if available.

Let’s compile the modified program:

```
$ egc
$ arm-none-eabi-size cortexm0.elf 
   text    data     bss     dec     hex filename
  15456     320     248   16024    3e98 cortexm0.elf

```

As you can see,  _io.WriteString_  causes a significant increase in the size of the binary: 15776 - 12964 = 2812 bytes. There isn’t too much space left on the Flash. What caused such a drastic increase in size?

Using the command:

```
arm-none-eabi-nm --print-size --size-sort --radix=d cortexm0.elf

```

we can print all symbols ordered by its size for both cases. By filtering and analyzing the obtained data (awk, diff) we can find about 80 new symbols. The ten largest are:

```
> 00000062 T stm32$hal$usart$Driver$DisableRx
> 00000072 T stm32$hal$usart$Driver$RxDMAISR
> 00000076 T internal$Type$Implements
> 00000080 T stm32$hal$usart$Driver$EnableRx
> 00000084 t errors$New
> 00000096 R $8$stm32$hal$usart$Driver$$
> 00000100 T stm32$hal$usart$Error$Error
> 00000360 T io$WriteString
> 00000660 T stm32$hal$usart$Driver$Read

```

So, even though we don’t use the  _usart.Driver.Read_  method it was compiled in, same as  _DisableRx_ ,  _RxDMAISR_ ,  _EnableRx_  and other not mentioned above. Unfortunately, if you assign something to the interface, its full method set is required (with all dependences). This isn’t a problem for a large programs that use most of the methods anyway. But for our simple one it’s a huge burden.

We’re already close to the limits of our MCU but let’s try to print some numbers (you need to replace  _io_ package with  _strconv_  in  _import_  section):

```
func main() {
	a := 12
	b := -123

	tts.WriteString("a = ")
	strconv.WriteInt(tts, a, 10, 0, 0)
	tts.WriteString("\r\n")
	tts.WriteString("b = ")
	strconv.WriteInt(tts, b, 10, 0, 0)
	tts.WriteString("\r\n")

	tts.WriteString("hex(a) = ")
	strconv.WriteInt(tts, a, 16, 0, 0)
	tts.WriteString("\r\n")
	tts.WriteString("hex(b) = ")
	strconv.WriteInt(tts, b, 16, 0, 0)
	tts.WriteString("\r\n")
}

```

As in the case of  _io.WriteString_  function, the first argument of the  _strconv.WriteInt_  is of type  _io.Writer_ .

```
$ egc
/usr/local/arm/bin/arm-none-eabi-ld: /home/michal/firstemgo/cortexm0.elf section `.rodata' will not fit in region `Flash'
/usr/local/arm/bin/arm-none-eabi-ld: region `Flash' overflowed by 692 bytes
exit status 1

```

This time we’ve run out of space. Let’s try to slim down the information about types:

```
$ cd $HOME/emgo
$ ./clean.sh
$ cd $HOME/firstemgo
$ egc -nf -nt
$ arm-none-eabi-size cortexm0.elf
   text    data     bss     dec     hex filename
  15876     316     320   16512    4080 cortexm0.elf

```

It was close, but we fit. Let’s load and run this code:

```
a = 12
b = -123
hex(a) = c
hex(b) = -7b

```

The  _strconv_  package in Emgo is quite different from its archetype in Go. It is intended for direct use to write formatted numbers and in many cases can replace heavy  _fmt_  package. That’s why the function names start with  _Write_  instead of  _Format_  and have additional two parameters. Below is an example of their use:

```
func main() {
	b := -123
	strconv.WriteInt(tts, b, 10, 0, 0)
	tts.WriteString("\r\n")
	strconv.WriteInt(tts, b, 10, 6, ' ')
	tts.WriteString("\r\n")
	strconv.WriteInt(tts, b, 10, 6, '0')
	tts.WriteString("\r\n")
	strconv.WriteInt(tts, b, 10, 6, '.')
	tts.WriteString("\r\n")
	strconv.WriteInt(tts, b, 10, -6, ' ')
	tts.WriteString("\r\n")
	strconv.WriteInt(tts, b, 10, -6, '0')
	tts.WriteString("\r\n")
	strconv.WriteInt(tts, b, 10, -6, '.')
	tts.WriteString("\r\n")
}

```

There is its output:

```
-123
  -123
-00123
..-123
-123  
-123  
-123..

```

### Unix streams and Morse code

Thanks to the fact that most of the functions that write something use  _io.Writer_  instead of concrete type (eg.  _FILE_  in C) we get a functionality similar to  _Unix streams_ . In Unix we can easily combine simple commands to perform larger tasks. For example, we can write text to the file this way:

```
echo "Hello, World!" > file.txt

```

The `>` operator writes the output stream of the preceding command to the file. There is also `|`operator that connects output and input streams of adjacent commands.

Thanks to the streams we can easily convert/filter output of any command. For example, to convert all letters to uppercase we can filter the echo’s output through  _tr_  command:

```
echo "Hello, World!" | tr a-z A-Z > file.txt

```

To show the analogy between  _io.Writer_  and Unix streams let’s write our:

```
io.WriteString(tts, "Hello, World!\r\n")

```

in the following pseudo-unix form:

```
io.WriteString "Hello, World!" | usart.Driver usart.USART1

```

The next example will show how to do this:

```
io.WriteString "Hello, World!" | MorseWriter | usart.Driver usart.USART1

```

Let’s create a simple encoder that encodes the text written to it using Morse coding:

```
type MorseWriter struct {
	W io.Writer
}

func (w *MorseWriter) Write(s []byte) (int, error) {
	var buf [8]byte
	for n, c := range s {
		switch {
		case c == '\n':
			c = ' ' // Replace new lines with spaces.
		case 'a' <= c && c <= 'z':
			c -= 'a' - 'A' // Convert to upper case.
		}
		if c < ' ' || 'Z' < c {
			continue // c is outside ASCII [' ', 'Z']
		}
		var symbol morseSymbol
		if c == ' ' {
			symbol.length = 1
			buf[0] = ' '
		} else {
			symbol = morseSymbols[c-'!']
			for i := uint(0); i < uint(symbol.length); i++ {
				if (symbol.code>>i)&1 != 0 {
					buf[i] = '-'
				} else {
					buf[i] = '.'
				}
			}
		}
		buf[symbol.length] = ' '
		if _, err := w.W.Write(buf[:symbol.length+1]); err != nil {
			return n, err
		}
	}
	return len(s), nil
}

type morseSymbol struct {
	code, length byte
}

//emgo:const
var morseSymbols = [...]morseSymbol{
	{1<<0 | 1<<1 | 1<<2, 4}, // ! ---.
	{1<<1 | 1<<4, 6},        // " .-..-.
	{},                      // #
	{1<<3 | 1<<6, 7},        // $ ...-..-

	// Some code omitted...

	{1<<0 | 1<<3, 4},        // X -..-
	{1<<0 | 1<<2 | 1<<3, 4}, // Y -.--
	{1<<0 | 1<<1, 4},        // Z --..
}

```

You can find the full  _morseSymbols_  array [here][11]. The `//emgo:const` directive ensures that  _morseSymbols_ array won’t be copied to the RAM.

Now we can print our sentence in two ways:

```
func main() {
	s := "Hello, World!\r\n"
	mw := &MorseWriter{tts}

	io.WriteString(tts, s)
	io.WriteString(mw, s)
}

```

We use the pointer to the  _MorseWriter_  `&MorseWriter{tts}` instead os simple `MorseWriter{tts}` value beacuse the  _MorseWriter_  is to big to fit into an interface variable.

Emgo, unlike Go, doesn’t dynamically allocate memory for value stored in interface variable. The interface type has limited size, equal to the size of three pointers (to fit  _slice_ ) or two  _float64_  (to fit  _complex128_ ), what is bigger. It can directly store values of all basic types and small structs/arrays but for bigger values you must use pointers.

Let’s compile this code and see its output:

```
$ egc
$ arm-none-eabi-size cortexm0.elf
   text    data     bss     dec     hex filename
  15152     324     248   15724    3d6c cortexm0.elf

```

```
Hello, World!
.... . .-.. .-.. --- --..--   .-- --- .-. .-.. -.. ---.

```

### The Ultimate Blinky

The  _Blinky_  is hardware equivalent of  _Hello, World!_  program. Once we have a Morse encoder we can easly combine both to obtain the  _Ultimate Blinky_  program:

```
package main

import (
	"delay"
	"io"

	"stm32/hal/gpio"
	"stm32/hal/system"
	"stm32/hal/system/timer/systick"
)

var led gpio.Pin

func init() {
	system.SetupPLL(8, 1, 48/8)
	systick.Setup(2e6)

	gpio.A.EnableClock(false)
	led = gpio.A.Pin(4)

	cfg := gpio.Config{Mode: gpio.Out, Driver: gpio.OpenDrain, Speed: gpio.Low}
	led.Setup(&cfg)
}

type Telegraph struct {
	Pin   gpio.Pin
	Dotms int // Dot length [ms]
}

func (t Telegraph) Write(s []byte) (int, error) {
	for _, c := range s {
		switch c {
		case '.':
			t.Pin.Clear()
			delay.Millisec(t.Dotms)
			t.Pin.Set()
			delay.Millisec(t.Dotms)
		case '-':
			t.Pin.Clear()
			delay.Millisec(3 * t.Dotms)
			t.Pin.Set()
			delay.Millisec(t.Dotms)
		case ' ':
			delay.Millisec(3 * t.Dotms)
		}
	}
	return len(s), nil
}

func main() {
	telegraph := &MorseWriter{Telegraph{led, 100}}
	for {
		io.WriteString(telegraph, "Hello, World! ")
	}
}

// Some code omitted...

```

In the above example I omitted the definition of  _MorseWriter_  type because it was shown earlier. The full version is available [here][12]. Let’s compile it and run:

```
$ egc
$ arm-none-eabi-size cortexm0.elf
   text    data     bss     dec     hex filename
  11772     244     244   12260    2fe4 cortexm0.elf

```

![Ultimate Blinky](https://ziutek.github.io/images/mcu/f030-demo-board/morse.png)

### Reflection

Yes, Emgo supports [reflection][13]. The  _reflect_  package isn’t complete yet but that what is done is enough to implement  _fmt.Print_  family of functions. Let’s see what can we do on our small MCU.

To reduce memory usage we will use [semihosting][14] as standard output. For convenience, we also write simple  _println_  function which to some extent mimics  _fmt.Println_ .

```
package main

import (
	"debug/semihosting"
	"reflect"
	"strconv"

	"stm32/hal/system"
	"stm32/hal/system/timer/systick"
)

var stdout semihosting.File

func init() {
	system.SetupPLL(8, 1, 48/8)
	systick.Setup(2e6)

	var err error
	stdout, err = semihosting.OpenFile(":tt", semihosting.W)
	for err != nil {
	}
}

type stringer interface {
	String() string
}

func println(args ...interface{}) {
	for i, a := range args {
		if i > 0 {
			stdout.WriteString(" ")
		}
		switch v := a.(type) {
		case string:
			stdout.WriteString(v)
		case int:
			strconv.WriteInt(stdout, v, 10, 0, 0)
		case bool:
			strconv.WriteBool(stdout, v, 't', 0, 0)
		case stringer:
			stdout.WriteString(v.String())
		default:
			stdout.WriteString("%unknown")
		}
	}
	stdout.WriteString("\r\n")
}

type S struct {
	A int
	B bool
}

func main() {
	p := &S{-123, true}

	v := reflect.ValueOf(p)

	println("kind(p) =", v.Kind())
	println("kind(*p) =", v.Elem().Kind())
	println("type(*p) =", v.Elem().Type())

	v = v.Elem()

	println("*p = {")
	for i := 0; i < v.NumField(); i++ {
		ft := v.Type().Field(i)
		fv := v.Field(i)
		println("  ", ft.Name(), ":", fv.Interface())
	}
	println("}")
}

```

The  _semihosting.OpenFile_  function allows to open/create file on the host side. The special path  _:tt_ corresponds to host’s standard output.

The  _println_  function accepts arbitrary number of arguments, each of arbitrary type:

```
func println(args ...interface{})

```

It’s possible because any type implements the empty interface  _interface{}_ . The  _println_  uses [type switch][15] to print strings, integers and booleans:

```
switch v := a.(type) {
case string:
	stdout.WriteString(v)
case int:
	strconv.WriteInt(stdout, v, 10, 0, 0)
case bool:
	strconv.WriteBool(stdout, v, 't', 0, 0)
case stringer:
	stdout.WriteString(v.String())
default:
	stdout.WriteString("%unknown")
}

```

Additionally it supports any type that implements  _stringer_  interface, that is, any type that has  _String()_ method. In any  _case_  clause the  _v_  variable has the right type, same as listed after  _case_  keyword.

The `reflect.ValueOf(p)` returns  _p_  in the form that allows to analyze its type and content programmatically. As you can see, we can even dereference pointers using `v.Elem()` and print all struct fields with their names.

Let’s try to compile this code. For now let’s see what will come out if compiled without type and field names:

```
$ egc -nt -nf
$ arm-none-eabi-size cortexm0.elf 
   text    data     bss     dec     hex filename
  16028     216     312   16556    40ac cortexm0.elf

```

Only 140 free bytes left on the Flash. Let’s load it using OpenOCD with semihosting enabled:

```
$ openocd -d0 -f interface/stlink.cfg -f target/stm32f0x.cfg -c 'init; program cortexm0.elf; arm semihosting enable; reset run'
Open On-Chip Debugger 0.10.0+dev-00319-g8f1f912a (2018-03-07-19:20)
Licensed under GNU GPL v2
For bug reports, read
        http://openocd.org/doc/doxygen/bugs.html
debug_level: 0
adapter speed: 1000 kHz
adapter_nsrst_delay: 100
none separate
adapter speed: 950 kHz
target halted due to debug-request, current mode: Thread 
xPSR: 0xc1000000 pc: 0x08002338 msp: 0x20000a20
adapter speed: 4000 kHz
** Programming Started **
auto erase enabled
target halted due to breakpoint, current mode: Thread 
xPSR: 0x61000000 pc: 0x2000003a msp: 0x20000a20
wrote 16384 bytes from file cortexm0.elf in 0.700133s (22.853 KiB/s)
** Programming Finished **
semihosting is enabled
adapter speed: 950 kHz
kind(p) = ptr
kind(*p) = struct
type(*p) = 
*p = {
   X. : -123
   X. : true
}

```

If you’ve actually run this code, you noticed that semihosting is slow, especially if you write a byte after byte (buffering helps).

As you can see, there is no type name for `*p` and all struct fields have the same  _X._  name. Let’s compile this program again, this time without  _-nt -nf_  options:

```
$ egc
$ arm-none-eabi-size cortexm0.elf 
   text    data     bss     dec     hex filename
  16052     216     312   16580    40c4 cortexm0.elf

```

Now the type and field names have been included but only these defined in  ~~_main.go_  file~~  _main_  package. The output of our program looks as follows:

```
kind(p) = ptr
kind(*p) = struct
type(*p) = S
*p = {
   A : -123
   B : true
}

```

Reflection is a crucial part of any easy to use serialization library and serialization ~~algorithms~~ like [JSON][16]gain in importance in the IOT era.

This is where I finish the second part of this article. I think there is a chance for the third part, more entertaining, where we connect to this board various interesting devices. If this board won’t carry them, we replace it with something a little bigger.

--------------------------------------------------------------------------------

via: https://ziutek.github.io/2018/04/14/go_on_very_small_hardware2.html

作者：[Michał Derkacz ][a]
译者：[译者ID](https://github.com/译者ID)
校对：[校对者ID](https://github.com/校对者ID)

本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出

[a]:https://ziutek.github.io/
[1]:https://ziutek.github.io/2018/04/14/go_on_very_small_hardware2.html
[2]:https://ziutek.github.io/2018/03/30/go_on_very_small_hardware.html
[3]:https://golang.org/doc/effective_go.html#interfaces
[4]:https://research.swtch.com/interfaces
[5]:https://blog.golang.org/laws-of-reflection
[6]:https://github.com/texane/stlink
[7]:http://www.world-semi.com/solution/list-4-1.html
[8]:https://en.wikipedia.org/wiki/1-Wire
[9]:https://github.com/npat-efault/picocom
[10]:https://github.com/ziutek/emgo/blob/master/egpath/src/stm32/examples/f030-demo-board/usart/main.go
[11]:https://github.com/ziutek/emgo/blob/master/egpath/src/stm32/examples/f030-demo-board/morseuart/main.go
[12]:https://github.com/ziutek/emgo/blob/master/egpath/src/stm32/examples/f030-demo-board/morseled/main.go
[13]:https://blog.golang.org/laws-of-reflection
[14]:http://infocenter.arm.com/help/topic/com.arm.doc.dui0471g/Bgbjjgij.html
[15]:https://golang.org/doc/effective_go.html#type_switch
[16]:https://en.wikipedia.org/wiki/JSON