In the last two articles on Forth, I’ve ranted about how it’s stunning but strange, and then gotten you set up on a basic system and blinked some LEDs. and while I’ve pointed you at the multitasker, we haven’t made much real use of it yet. getting started on a Forth system like this is about half the battle. working inside the microcontroller is different from compiling for the microcontroller, and figuring out the workflow, how to technique problems, and where the beneficial resources are isn’t necessarily obvious. Plus, there’s some terrific features of Mecrisp-Stellaris Forth that you might not notice until you’ve hacked on the system for a while.
Ideally, you’d peek over the carry of someone doing their thing, and you’d see some of how they work. That’s the goal of this piece. If you’ve already flashed in our version of Mecrisp-Stellaris-plus-Embello, you’re ready to follow along. If not, go back and do your homework real quick. We’ll still be here when you’re done. A lot of this post will be very certain to the Mecrisp-Stellaris flavor of Forth, but given that it runs on tons of ARM chips out there, this isn’t a bad place to be.
Getting Acclimatized
The first thing you’re going to need to get used to in Forth is the stack. You know that old chestnut about people only being able to keep five (seven?) things in their mind at one time? Forth puts that to the test.
Last time, I briefly pointed out the .s (“print stack”) command. In the Hackaday Edition, I’ve re-defined the conventional Mecrisp .s to be a little bit less verbose, and to my eyes a lot more readable. If you find yourself hitting .s a lot, and you will, I’ve also written a function that (temporarily) overwrites the “ok.” prompt by appending a stack printout to it, whenever you hit enter. type print.stack and hit enter one a lot more time to see how it works. hitting the reset button, or typing reset will wipe everything in RAM, and that includes the stack-printing prompt, so you’ll be back to a clean slate.
Now is probably a good time to play around with the stack operators. have you read the Mecrisp glossary? check out the list of stack juggling operations there. turn on print.stack and play around until they all make sense.
Have you run words yet? It spits out a linked list of every word that Forth knows, along with the memory locations where they live, and some extra details — too much detail, unless you’re debugging the system itself. There’s an extra, nonstandard, word in Mecrisp that just prints out the function names: list. give that a shot now. If you haven’t already defined a few words, do so.
: hw .” hello, world!” cr ;
is a good one to have on hand.
Layers in Memory
Mecrisp-Stellaris’ memory is divided into two gross locations: RAM and flash. All words that are provided before the “— Mecrisp-Stellaris Core —” mark are in RAM, and will be lost on reset or power-down. new RAM functions will be appended to the front of the list.
After the “— Mecrisp-Stellaris Core —” mark come functions in flash. In the early parts of flash, before “— Flash Dictionary —” is the conventional Mecrisp Forth core. From there until “<>” are words taken from the Mecrisp distribution that are normally useful, including some debugging functions and multitasking. preceding “<>” are the contributions from the Embello libraries, including a lot of GPIO definitions, and those before “<>” were added just for this post series.
What’s not evident is that all of these markers with brackets surrounding them are cornerstones. These allow you clear out flash up until that memory location. So if you’ve added some extra functions into flash, and want to clear back out to the Hackaday edition default state, you can type <
Overwriting History
If you define a word twice with the same name, you’ll have two versions of the word in the dictionary. When a word is called or compiled, the interpreter looks through memory, from the top of RAM down, and then from the end of flash back to the beginning. Words with the same name in RAM thus get called before those in flash. What can be particularly odd about Forth is that, because it compiles in real-time, the word that is referred to in any calling word is the one that was on the top at the time the calling word was defined. Making tangled histories is a sure way to go insane.
: foo .” foo!” ;
: bar foo .” bar!” ;
bar foo! Barra! OK.
: foo .” bizzle!” ; Redefine foo. OK.
foo bizzle! OK.
bar foo! Barra! OK.
On the other hand, here’s a great way to work that takes advantage of these various memory features. RAM gets erased on every reset, giving you a clean slate, but by using cornerstones, flash isn’t immutable either. Of course, the deeper in flash you have to erase,the a lot more words you have to redefine later, assuming that some of them were useful. This suggests a layered technique to development, with the most “core” words innermost in flash. That’s also natural because words need to be defined to be called, so defining the basics first makes sense anyway.
You can compile a new word to RAM (the default) by calling compiletoram or to flash by invoking compiletoflash. Prototype your words in RAM. feel complimentary to overwrite them as lots of times as you want, but remember that you have to redefine any dependent words after you change something upstream to keep the calling history intact. once you’re made with a chunk of development, type reset and clear RAM. now redefine these words into flash. If you develop your application from the bottom up, you’ll find that this all hangs together nicely. When you’ve discovered a bug in something that’s already written to flash, the cornerstones come to the rescue.
Finally, this layered nature of Forth definitions can be really handy. For example, a function init that’s defined in flash gets run on each reset. It includes things like setting the processor speed and the system tick, that you probably don’t want to mess with. because you can overwrite init, and any compilation uses the words available at the time of compilation, you can simply layer your functionality onto init: : init init .” howdy!” cr ;. The first “init” is the name of the new word, and the second is calling the pre-existing init, and doing all of the setup. The remainder of the definition is yours to play with. On restart, everything will get carried out in the buy it was defined.
Criatura Confortável
I write a lot of code in an editor (Vim) and then send it over to the chip to play around with. very specifically, I wrote a script called forth_upload.sh that contains the following:
[[ $1 ]] && TERM=$1 || TERM=/dev/ttyUSB0
DELAY=0.2
while read -r f; do echo “$f” ; echo “$f” > $TERM ; sleep ${DELAY}s ; feito
To send a file to the serial port, and thus to Forth, forth_upload.sh < myfunctions.fs will work. It loops through each line in the file, allowing a 0.2 second delay for the system to compile anything. This delay is too long, in my experience, but the bugs from having it short are lousy to find. Folie takes the technique of trying to find the “ok.” prompt to speed things up, but this has issues of its own. If you’re using Vim, you can send individual words or a full file with the following:
nnoremap
inoremap
nnoremap
This setup lets me develop and tweak functions in an editor that I like, and then send them nearly quickly into the Forth system to test out. I keep a terminal window open that’s always logged into the Forth system, so I can enjoy the new words get defined in, and then start playing around with them interactively. It’s pretty sweet.
The Solar Cell Tester
Now to a quick real example that will make use of all of the above. I recently made a decision to characterize a bunch of small solar panels that I had in my junk drawer. This indicates adjusting the load on the panel and noting down the voltage generated by the panel and the current through the load. I hooked up two multimeters and wrote down some numbers, but this is undoubtedly a job for a microcontroller.
As is clear from the image, this is a quick lashup. The larger chunk of copper-clad has a current-measurement resistor and a pair of resistors configured as a voltage divider to step down the panel voltage to something the 3.3 V ADC can handle.
Dangling off that is the (silvered) variable load circuit, which is entirely sub-optimal: a MOSFET dissipates the heat by being turned half on by a PWM’ed voltage fiiltered by that 100 uF capacitor. The 9 V battery and optoisolator were cobbled on to make sure a high enough voltage to fully open the MOSFET, which wanted a lot more than 3.3 V. This horrible, but functional, load circuit was a later addition — the first version just had a potentiometer here, until that got smoked by running too much current through it.
The pushbuttons are used to start and stop recordings from the device out on the balcony without having to run back inside. The procedure was to alligator-clip in a new panel, and hold the white button while it made recordings. The small black button is pressed once to demarcate a new cell’s data. The data goes back to my laptop over UART serial through an ESP8266 running esp-link as a transparent WiFi serial bridge, seen in the upper left, ideal next to the recycled laptop batteries. hot glue and cardboard round out the state-of-the-art build.
To the Datasheet!
This kind of quick and hands-on tool-building is where Forth shines. The Jeelabs Forth libraries already have some functions that simplify the ADC setup, but they actually didn’t work for me, so I looked to the datasheet.O mínimo que se precisa para obter o funcionamento do ADC é ligar os relógios periféricos ADC, ativar a unidade ADC e, em seguida, selecione o tempo de amostragem do ADC. Também podemos querer executar o procedimento de calibração do ADC para garantir que nossas leituras estejam corretas. Este é todo o tipo de detalhe de baixo nível que você teria que fazer com qualquer microcontrolador – ou inclinado em uma biblioteca que faça isso para você.
Lendo a folha de dados, para ativar o relógio do sistema ADC, precisamos definir o bit adc1en no registro de memória do RCC-APB2ENR. Qualquer que seja. O Mecrisp-Stellaris possui algum código de suporte que lê esses valores de arquivos fornecidos pelo fabricante. Eu incluí-los no núcleo / registradores / diretório. O arquivo de mapa de memória tinha grandes mnemônicos para todos os registros, mas não estou emocionado com a maneira como os nomes de bits individuais são tratados. Isso é um yak para se barbear em outro dia, estou tentando fazer coisas.
Aqui está o conjunto de palavras ADC de nível de bits que eu inventei:
\ Ligar
\ Pb0 – AIN8 está conectado à tensão do painel
\ Pb1 – AIN9 está conectado ao resistor de sentido atual
: adc.rcc-enable 9 bit rcc-apb2enr bis! ; \ set adc1en.
: adc.set-adon 0 bit adc1-cr2 bis! ; \ Definir Adon para ativar o ADC
: adc.set-cal-bit 2 bits adc1-cr2 bis! ;
: adc.cal-feito? 2 bits adc1-cr2 bit @ 0 =;
: adc.isdone? 1 bits adc1-sr bit @;
9 bit cria um número com o nono conjunto de bits e todos os outros zero e rcc-apb2enr bis! Define o bit desejado no registro de controle ADC relevante. Tudo em tudo, bonito opaco, mas também um para um com as instruções da folha de dados. Boa nomeação dessas palavras de camadas de fundo torna as coisas um pouco melhor uma camada. Por exemplo, a palavra responsável por executar uma calibração (defina um bit, aguarde até que outra seja clara) é relativamente legível:: adc.cal adc.set-cal-bit iniciar adc.cal – feito? até ;.
E aqui vemos nosso primeiro controle de fluxo, o começo … até construir. A compra é estranha, certo? começar inicia o loop, a função de teste adc-cal-feita? Executa, deixando um valor verdadeiro ou falso na pilha e, em seguida, até ler este valor e voltas de volta para começar até que o valor seja verdadeiro. Isto é o mesmo que um c enquanto (! Adc-cal-feito ()) {;} loop de espera ocupado. E é uma introdução a outra idiom: usando? para qualquer coisa que retorne um booleano.
Teste, corrigir e teste novamente
Então vamos testar essas palavras. Inicializando o ADC e a leitura é fácil: Agora vamos levá-los para uma corrida e certifique-se de que os valores são o que estamos esperando. Chicotando um ambiente de teste rápido em frente é um dos seus principais pontos fortes. : TEST-ADC COMEÇAR ADC @. CR 10 ms chave? até ; Define uma palavra que lê o ADC, imprime o valor e repete a cada dez milissegundos até que você pressione ENTER. Se você fizer isso com um painel solar sob uma luz fluorescente, por exemplo, você pegará as ondulações de frequência da rede, pois eles bateram no painel, o que é algo que você pode não ter pensado. A capacidade de brincar com suas novas palavras assim que você as definir torna esse tipo de fluxo de codificação investigativa naturalmente.
Tempo verdadeiro da história. Quando eu estava escrevendo este código, eu havia definido um tempo de amostra muito curto nas entradas do ADC. Isso faz com que o valor de uma leitura afetasse a próxima, porque o capacitor interno do ADC não tem tempo para carregar ou descarregar totalmente. Eu estava recebendo leituras muito altas para a corrente quando a tensão era alta, e leituras muito baixas para corrente quando a tensão estava baixa. Eu depurei isso bastante rapidamente, escrevendo uma rotina que leia cada canal algumas vezes seguidas e imprimi os valores, e deixei essas palavras para a posteridade.
(Tome 8 amostras de cada, para depuração)
: Leia-ambos 8 0 Read-v. Loop 8 0 Read-i. ciclo ;
: Readloop inicie a leitura-ambos CR 100 ms da CR? até ;
O feedback deste exercício de teste leva-me a trocar os tempos da amostra no ADC ao seu valor máximo, porque nada aqui era vital e eu estava usando uma fonte bastante de alta impedância. (O padrão é amostrar tão rapidamente quanto p