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In The Air: Episode 6 - Getting Ready For PCBs

michaelwylie

Previous Posts:

In The Air: Epidose 1: Introduction

In The Air: Episode 2 - Preparing for Surface Mount Work

In The Air: Episode 3 - Surface Mount Beginnings

In The Air: Episode 4 - Inductors

In The Air: Episode 5 - PCB Design

 

Update

This week involved procuring parts. I contacted Wurth for the following parts:

 

 

The first two kits are for the 0805 and 1206 capacitors I used on my board. The third item is a 2 position terminal block for a power supply input. Items 4 and 5 are male and female headers for the TI Booster Pack Interface. I used all Texas Instruments ICs on my board, so I contacted Christian (@doctorcdf) and he was able to procure samples from TI for me. For everything else, I used my budget from Newark. So, while I wait for everything to be delivered I began working with the CC3200 Launch Pad. I'm following the tutorials (first, second) created by @shabaz for the CC3200.

 

This leaves me with not much to talk about for this week's blog post. How about some more about PCB design, and then a little about reading? Figure 1 shows a zoomed portion of my Booster Pack design. U8, down at the bottom left, is a TINY humidity and temperature sensor. I was surprised that no one who read my last post challenged me on the layout decision for that chip.

 

image

Figure 1: Zoomed picture of my Booster Pack design.

 

When placing the sensor directly on my main PCB like this, it should be thermally isolated from the rest of the board. Figure 2 shows how TI suggests doing this in the HDC1000 datasheet. The white portions are slots drilled in the PCB, and this reduces the thermal mass. What's that all about? Well, for our purposes, a low thermal mass allows the sensor to respond quickly to temperature fluctuations. What I'm really getting at here, is that I didn't include any slots in my PCB design.

 

image

Figure 2: Isolating the HDC1000 and reducing its thermal mass.

 

Why, oh why would I have done this? I've got three reasons:

  1. I don't want to measure the ambient air temperature. I want to know the board temperature. The microcontroller might have a temperature sensor built-in, but that's on the launchpad not the Booster Pack.
  2. Slotted holes are always a pain when getting your PCB fabricated. I'm not sure why this is an issue, but every time I've gotten quotes for PCBs the manufacturers were more concerned with how many slotted holes the board contained and not the overall hole count.
  3. I don't even have a good reason to have this chip included, I just thought it would be fun to put it on the board.

 

I suppose this brings me to the section where I talk about reading. The more I mentor students, the more I reflect on my own education and experience. Inevitably, when I first started with a student we would have a conversation something like this:

 

     Student: Hey Mike, I can't seem to get XYZ working.

     Mike: What does the datasheet say about it?

     Student: Ahhh, I dunno.

 

I think this is a symptom of a larger problem: students can "read" without understanding. I'm guilty of it myself. I can easily recall reading technical documents and stopping only to think, "I haven't understood anything I've read for the last page and a half". Not only did I not understand it, I don't even remember what I read! It's like my "reading" was on autopilot. When I was a teenager, my father always told me RTFQ², which stood for Read the (expletive deleted) question squared, or in this case twice. Although I prefer a cleaner version, I think the principle remains true; reading and understanding is important. I have two techniques I use to help me understand what I am reading. The first is to periodically stop and ponder what I have read. It's amazing how effective this technique can be. The second is to read with a pen and paper handy to make notes about what I've read. I only discovered how helpful this was when I started preparing lectures as a graduate student.

 

Hope your projects are going well! Til next time.