Arduino Serial Queue

Question

So I am programming an LED Music Visualizer that uses a combination of Processing and Arduino. I have it all working as I want to but there is a small problem. If I were to stop the music, then the Arduino will keep displaying the visualization for a few seconds. I believe that this issue would be caused by it "catching up" on new serial data from Processing like a queue. So my question is: When arduino uses the Serial.read() function, is it using the most recent value or is it using the next value received after the last one that it used, like a queue of values.

Example: Processing sends the values 1,2,3,4,5,6,7,8,9 over serial in 10 millisecond intervals. Arduino only performs Serial.read at 20 millisecond intervals. Would the arduino read the values 1,3,5,7,9 in or would it read the values 1-9 but in twice the time?

Sorry for the confusing question, hopefully somebody could shed some light. At the core, it is a question on how serial works, you can ignore the part about the music visualizer.

Answer 1

It sounds like you figured most of your question out already.

(Someone correct me here if I'm wrong). The Atmega 328p uC, I believe, has an 8 byte hardware buffer for RX serial data. In software, the Arduino core code defines a maximum ring buffer of 64 bytes that acts as a FIFO queue.

So, what does his mean? Let's just focus on the hardware buffer since it's fairly easy. Say you send your data 1,2,...,10 at 10ms intervals, and you read your data at 100ms intervals (for illustration purposes). The problem is that you send 10 bytes of data but only have room for 8 bytes. So, your first two values (1,2) are pushed out of the buffer and lost. This idea also illustrates a FIFO queue, first in first out.

The same idea can be extended to a ring buffer.

Anyway, like you said, this could explain your problem. If you have 64 bytes, which will obviously be full since your inevitably losing data (20ms read with a 10ms write), you'll be seeing the effects for 64 * 20ms = 1.28s. There's probably some processing delay too which will extend this delay, but it's more than likely minimal if it's efficient code.

Answer 2

Arduino only performs Serial.read at 20 millisecond intervals.

It sounds like you're using delay to pace the reads. But why? You'll start to lose data after reading just 3 or 4 characters (60ms to 80ms). Serial at 9600 can send 1 character per millisecond, 20 times faster than you are reading them.

If you edit your post to include your code, we might be able to make a suggestion to eliminate that delay.