I have programmed Atmel processors for years using the ICC AVR C complier but am new to Arduino.
How do I program a hardware serial interrupt in Arduino? An interrupt for each character received. Polling is no good - too slow and misses characters.
Here's how the interrupt service must work.
A command block contains length byte after the SOT byte.
The interrupt service routine holds onto that value and increments the receive byte
count for each subsequent byte received.
When the receive count equals the message length value, a flag byte is set and the main
loop now acts on the command received.
So the main loop doesn't deal with the byte reception. The ISR handles that.
I have done this many times with the ICC compiler but not on Arduino.
You might want to go looking for a little gem called CMRI by madleech - it's a reception routine for a common model railroading communication protocol, but it contains the elements you're looking for.
Normally, you don't. The Arduino Core already includes an interrupt driven UART driver, so it shouldn't miss characters, unless your message size exceeds the buffer size (64 bytes, on an Uno or similar AVR.)
There may be certain circumstances (packet-like protocols) where you'd want to replace the default driver with your own, but it would get somewhat messy (most of the mess would be making sure that the Arduino code is left out; an interrupt driven UART driver would look like normal avr-gcc (which is not quite the same as ICC. (sigh. Richard is a friend of mine...))
(Using the standard Arduino Serial support makes you mostly board and CPU independent...)
1. Iif you enter A in the InputBox of the Serial Monitor and then press on the Send Buton, a 10-bit frame (1-StartBit, 8-bit Character Code, 1-STopBit) is transmitted towards ARDUNINO UNO at speed determined by Bd = 9600 (say).
2. The frame arrives at UNO's UART Receiver; where, Start and Stop bits are discarded, and then the remaining 8-bit Character code is automatically stored in the FIFO type Serial Buffer on interrupt basis in the background.
3. Now, the Programmer executes the following codes to read the arrived character from Buffer, save in a local variable, and then send it to the OutputBox of Serial Monitor.
void setup()
{
Serial.begin(9600):
}
void loop()
{
byte n = Serial.available(); //check that if a character has arrived
if(n != 0) //at least -character has arribed and saved in Buffer
{
char ch = Serial.read(); //read character from Buffer and save in ch
Serial.print(ch); //show the charcater on Serial Monitor
}
}
4. Create sketch to receive ASCII formatted decimal number 1234 from the InputBox of Serial Monitor, retrieve the decimal number and save in variable y, and then send it back to the OutputBox of the Serial Monitor.
Welcome to Arduino! To handle serial interrupts, you can use the USART_RX_vect interrupt, which triggers when a byte is received.
Enable the interrupt with sei(), then in the ISR, read the byte with UDR0, store it, and increment the byte count. When the count matches the length byte, set a flag for the main loop to process the data.
This way, the main loop stays free from serial handling, and the ISR does all the work. Let me know if you need more details or example code!
If you want to just read from (or write to) the UART there are much simpler ways than using the code provided in the core. The code will most likely not be as portable as using the Serial class since the register names may not be the same or the ISR vector names may differ (or some other reason) Also the buffer management will have to be done by you. The Serial class uses a 2 ring buffers.
I keep this example for myself in case i need it (for a 328P) this implements both a simple TX & RX
#define BAUD 250000UL
#define UB ((F_CPU / (8 * BAUD)) - 1)
void setup() {
// UART TX and RX pins
DDRD |= (1<<PORTD1) | (1<<PORTD2) ; // set TX pin and pin 2 to output
DDRD &= ~(1<<PORTD0); // set RX pin to input
//UART initialization,
UCSR0A = 0x02; // 1<<U2X | 0<<MPCM;
UCSR0B = 0x98; // 1<<RXCIE | 0<<TXCIE | 0<<UDRIE | 1<<RXEN | 1<<TXEN | 0<<UCSZ2; // enable TX & RX
UCSR0C = 0x06; //0<<UMSEL0 | 0<<UPM1 | 0<<UPM0 | 0<<USBS | 1<<UCSZ1 | 1<<UCSZ0 | 0<<UCPOL; // 8N1
UBRR0H = (UB >> 8); // set baud rate
UBRR0L = (UB & 0xFF); // HL register
}
void loop() {
/*const char * dat = "Hi There\n";
uart_write_string(dat);
delay(1000);*/
}
void uart_write(char data) {
UDR0 = data;
while((UCSR0A & 1<<TXC0)==0);
UCSR0A |= 1<<TXC0;
}
void uart_write_string(const char * data) {
for(uint8_t count = 0; data[count] != 0; count++) {
UDR0 = data[count];
while((UCSR0A & 1<<TXC0) == 0);
UCSR0A |= 1<<TXC0;
}
}
ISR(USART_RX_vect) {
unsigned char c = UDR0; // read the character from the register, this is all that is really required.
uart_write(c); // echo the character or you can store it in the buffer.
PORTD ^= (1<<PORTD2); // toggle pin 2 can be used to connect a LED 'optional' of course
}
So once the registers are set there isn't much code left to do what is required. For details look at the datasheet.
As long as the Serial class is not used in the same sketch at all there is no conflict, since the interrupt vector is only declared once.
n.b. I actually did the research with help from people on this forum when i was confronted with the older ATtiny core (1.5.2) for an ATtiny2313 where in that core the Serial class used up pretty much all of my memory.
Pretty much yes. The declaration of the object defines the ISR vector (in this case USART_RX_vect) and it can only be defined once.
For boards with multiple UARTs it is possible to use different methods for different ports since those are different vectors (eg Serial1.begin() has no effect on what you do with 'Serial' / UART0 )
OK, so you are saying the Uno has a 64 byte buffer by default? So I can just poll the buffer and process the bytes within a called subroutine? That should work as the time the subroutine takes to process the bytes would be roughly the same time the ISR takes to do the same.
The protocol requires an acknowledge (0x11) within 10ms after a packet is sent. If it does not get an acknowledge, it sends the packet again. If that fails to acknowledge it stops talking to, in this case, the Uno. Then the Uno would have to sign back on. Can't have that happening normally.
Just FYI for anyone who may have the experience, I am working with the Sony broadcast VTR protocol. This is the communication protocol for a controller to control a broadcast VTR. Basically the way an IR remote would control a VHS VCR.
The RS232 (actually RS422) parameters are 38.4k/O/8/1
Then you would have to constantly poll to make sure that you don't miss the acknowledge.
If you write you own ISR you can keep track of whatever needs to be done for the protocol, like counting the incoming bytes and deciding on5 a complete package and send the acknowledgement.
I think it is a better plan and easy enough to do. 10ms is not a lot of time, that is the transmission time of less than 3 bytes
at 38.4kbps. (rough math, 11 bits per byte, 1 start, 1 parity, 1 stop and 8 data bits )
Not quite !. You see there still an ISR doing the same thing already. The ISR_vector is triggered, the ISR is fired, the byte is copied into the buffer and the index is increased.
If you read() those bytes, you copy from the buffer and increase the other index (it is a 64 byte ring buffer after all)
The closest you can get is by constantly polling for the difference between those 2 indices, (available() )and react the moment that you have the whole package. Transmit the acknowledgement and read the package.
If you write the ISR, not only can you specify the buffer size, but you can manage and monitor the counter from within the ISR, so there is no need to poll and you program can go on doing what it, .. well, could be doing i guess. It may not even have other tasks i don't know.
Not really sure what that means.
But anyway, if you compare the counter, the moment that it is increased you will be able to respond quicker, and it is more efficient since there is one less stage of copying and counting.
And it really isn't very hard to do.
Consider the ISR from my example and modify it.
#define PACKAGE_SIZE 10
volatile uint8_t buf[PACKAGE_SIZE];
volatile bool acknowledged = false;
ISR(USART_RX_vect) {
static uint8_t ndx = 0;
buf[ndx] = UDR0; // read the character from the register, this is all that is really required.
ndx++;
if (ndx == PACKAGE_SIZE) {
ndx = 0;
acknowledged = true;
uart_write(0x11);
}
Now the only thing you need to do is read the package before it is overwritten by the next incoming bytes. But if you want to be sure you should probably use a double buffer. But write straight to it though, it will save time.
#define PACKAGE_SIZE 10
volatile uint8_t buf [2] [PACKAGE_SIZE];
volatile int8_t acknowledged = -1;
ISR(USART_RX_vect) {
static uint8_t ndx = 0, buffer = 0;
buf [buffer] [ndx] = UDR0; // read the character from the register, this is all that is really required.
ndx++;
if (ndx == PACKAGE_SIZE) {
ndx = 0;
acknowledged = buffer;
buffer = 1 - buffer;
uart_write(0x11);
}
}
Like this while the acknowledged == -1 nothing complete has been received and otherwise it will be the buffer.
That does leave us with the question, what happens if there is an in- or over- complete package
OK, I'll digest this in detail. I have been programming microprocessors for 40 years now starting with the Z80 using a PC-XT to write the code and cross assemble. Then came the first C cross compilers. in the mid 1980s. So complexity doesn't scare me.
The problem I have with the Arduino environment is that it's operating at a higher level than I am used to. The IR library for example is great, all the timings are worked out to the point where they self adjust to the processor you are using. But to use them, I must work within the bounds of the Arduino IDE, which I just need to learn more about.
I wrote an IR communication program in raw C years ago for an ATmega8515. It was quite an exercise programming the timer interrupts to get the pulse timing right.
program timers
compile and run
look at waveforms on a scope
tweak the code
repeat
Well yes, but the low-level stuff is still available and if you already know what processor you are using, you don't need to get the timing right for every single one. Still the IR-library is not what we are talking about here.
I was 10 when a started on zx81 basic, Z80 programming was not easy for me to get my head around, and with the IDE i had at the time and the lack of experience in organising i didn't actually write much. My passion for Low-level programming on a limited resource micro-processor is still there. Attiny13 has 1KB Flash and 64 bytes of RAM and i want to squeeze every bit out of it. The datasheet tends to provide quite a bit of info on the registers as well as small code examples. This also goes for the 328p.
I am still curious though is there a header in the data-stream. or is it break oriented. There must be a way in which the start of a packet is signalled to the receiver.
Yes, and that solves (probably) the "losing data" problem.
The down-side is that the Arduino Core buffering is strictly a byte FIFO, so to implement any sort of packet, message, or line-oriented behavior on top of it, you'll need additional buffering on the user-program side of things, which is somewhat depressing given the limited memory that is usuaully available...
