In which we discover some potential "gotchas" with regard to constructing a simple 14-band (2 x 7) audio spectrum analyzer using two MSGEQ7s and a chipKIT or Arduino microcontroller development platform.
Good grief! Where does the time go? It seems like only a couple of days ago that I was waffling on about the construction and testing of my MSGEQ7-Based DIY Audio Spectrum Analyzer. However, the sands of time have wended their way through the hourglass, as is their wont; the days have turned into weeks; and thus we find ourselves in the "here-and-now."
What can I say? I'm too young for all of this excitement (LOL). As usual, just to set the scene before we leap headfirst into the fray with gusto and abandon, let's first take a look at this short video to remind ourselves as to just how cool this looks in the real world.
Now, in this column, I'd like to share a few extra tidbits of trivia and nuggets of knowledge that I've picked up along the way. With regard to accessing the spectrum data from the MSGEQ7s, let’s start by reminding ourselves of the official timing requirements from the data sheet, as illustrated below.
As we see, a positive-going pulse on the RESET signal kicks everything off. This pulse takes a copy of the current peak detector outputs and stores (latches) the values. We then apply seven negative-going pulses to the STROBE input. Every time the STROBE input goes low, we can read the value of one of the bands on the DATA_OUT signal, starting with 63 Hz and working our to 16,000 Hz.
The DATA_OUT is an analog value whose magnitude reflects the value from the corresponding peak detector. This value can be read using one of your microcontroller's analog inputs. (Observe that the DATA_OUT signal is clamped to 0V when the STROBE signal is HIGH.)
When I originally purchased the MSGEQ7 chips, I looked around on the Internet to see if there was any sample code to start me on my way. In fact, I found quite a few different examples, each doing things in slightly different ways, but a representative version of the function used to read the spectrum data values from the MSGEQ7s might be as shown below.
(Click here to see a larger version with comments)
Note that the names I use for the RESET and STROBE signals in my code are "ctrlReset" and "ctrlStrobe," respectively. As we see, we start with a positive-going pulse on the ctrlReset signal, and then we enter a loop to read the seven data values. Observe that the only delay that is explicitly defined occurs after the ctrlStrobe signal is driven LOW. According to the data sheet, the minimum output settling time is 36 µs, and some of the examples I found used this exact value, but in my code I boosted this up to 40 µs, just to "make sure."
To be honest I feel like an old fool (but where are we going to find one at this time of the day? LOL). I know better than to simply use someone else's code without thinking about it, but that's what I did, and it worked, so I might have simply left it at that, but...
...as I was driving into work one morning, I suddenly got a "niggly feeling" about the width of the positive-going pulse on the ctrlReset line. From the data sheet, we know that this pulse has a minimum width of 100 ns. It struck me that all of the code examples I'd seen had been for an 8-bit Arduino Uno or Arduino Mega running at 12 MHz, but I'm running this on a 32-bit chipKIT MAX32 running at 80 MHz. Could it be that my pulse was too narrow?
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