On my most recent revisions of my breakout boards for the Atmega328p and Attiny85, I added a 6-pin ISP header. ISP stands for In System Programming, which, as the name suggests, means that the header is used to program the microcontroller as it sits in a circuit, which is especially handy for boards without a way to plug directly into your computer’s USB port or a board with a surface-mount microcontroller.
I’ve never actually used these headers since I was used to using my Arduino Uno to program the microcontroller for both of my breakout boards. To make sure I could add the 6-pin ISP header to the boards correctly, I read up on it from different sources to understand how to make the connections.
On the pinout of the microcontrollers, there are pins labelled SCK, MISO, and MOSI. These are three of the six pins of the 6-pin ISP header. The other three pins on the header are pins we all know well: Reset, Vcc, and Ground. The asterisk on my boards signify the first pin of the ISP header, which should be the MISO pin. The illustration of mine uses the pinout image in the Attiny datasheet where the six pins are underlined. I also drew up a simple diagram of the header pinout.
I received a cheap USBtinyISP board from China which took quite a while to get here. Thankfully it didn’t take much longer to get up and running.
The board came with a 6-pin ribbon cable and a USB cable. The USB cable is so short that it’s virtually unusable since the cable isn’t long enough to get the board to my desk from my computer. Thankfully, I have a longer cable I can use instead.
The first thing I did was to check the pinout of the cable so I don’t plug it in the wrong way on my boards. The easiest way the figure out the orientation of the header is to find Vcc and Ground with a multimeter, where you should see 5v across. It’s a 50/50 chance… so of course I got it wrong the first time as I saw the voltage reading fluctuate in the mV range.
Once I got the orientation of the connector right, I plugged it into one of my boards. I marked the first pin with a little sticker, as shown in the picture above.
Surprisingly, the Adafruit USBtinyISP drivers works with this board. I opened up Device Manager and updated the drivers with their files. The seller of this board had their own hosted driver files, though some poking around showed it was just the Adafruit drivers anyway.
First up was my Attiny85 Breakout Board. It’s not meant to fit into these half-sized breadboards but bending the power pins a little bit got it in. (If you bought one of these boards, don’t do this. This is a test unit, after all.)
I was able to upload the blink sketch directly from the Arduino IDE, with the Attiny85 settings and setting the programmer to “USBtinyISP “. The timing of the blink sketch was weird the first time I tried it, which made me realize I must have previously burnt the bootloader to use the 8MHz internal clock instead of the default 1MHz. I decided to try burning the bootloader so it would go back to using the 1Mhz clock. The USBtinyISP was able to do it and the sketch ran perfectly.Now for the Atmega328p Breakout Board. To upload through the USBtinyISP and the Arduino IDE, I can’t just click Upload like I did with the Attiny85 board. I have to hold shift when I click the Upload button to Upload Using Programmer. This is supposed to upload the sketch to the microcontroller without needing a bootloader. Once I upload a sketch with the USBtinyISP, I can’t upload a sketch if I place it in an Arduino Uno, so I guess that uploading with a programmer erases the bootloader…? To be able to use the microcontroller on an Arduino Uno again, I have to burn the bootloader with the USBtinyISP board. It’s not a big deal, but it’s something I have to remember to do so I don’t get confused why things aren’t working later on.
So with the FTDI and USBtinyISP programming tools at my disposal, I’m very excited to get the next revision of my Atmega328p Breakout Boards as they have headers for both devices. Stay tuned for news on that! Thanks for reading!