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New product: Pololu USB AVR Programmer v2.1
Our new programmer, the Pololu USB AVR Programmer v2.1, was supposed to be a minor update to our existing programmer, coming right after the A-Star 328PB Micro that we released last month, with the main point of excitement being the Las Vegas-inspired $7.77 price. But as we were testing the combination of the programmer with the A-Star, we were getting brown-out resets on the programmer when it powered the A-Star. The relevant part of the circuit was just a P-channel MOSFET that connected the programmer’s own logic voltage (which we call VDD) to the VCC pin of the ISP connector:
MOSFET-based target VCC power control used on Pololu USB AVR Programmer v2.
The problem was caused by the MOSFET turning on too well (quickly and with low resistance), causing the logic voltage on the programmer to drop if the VCC of the target device had more than a few µF of discharged capacitance on it. The bigger the capacitance on VCC, the bigger the voltage drop on VDD, until eventually the drop was big enough to trigger the brown-out reset protection on the programmer’s microcontroller. We tried various firmware tricks with our existing hardware, such as turning on the MOSFET for very short pulses to gradually charge up the target device’s VCC capacitance, but none of them worked reliably enough. So in the end, we decided to redo our PCB and put in a dedicated high-side power switch with a controlled slew rate. The new programmer can now power target boards with up to about 33 µF on their logic supplies.
These are the two other improvements we made to the new v2.1 programmer over the older v2 programmer:
- Plugging a v2 programmer into a 3pi robot could cause one of the motors to briefly run at full speed because the programmer’s circuitry for measuring VCC could inadvertently pull up one of the 3pi’s programming pins (which doubles as a motor driver input) before the GND connection was established. The v2.1 programmer has improved circuitry for measuring VCC which limits the duty cycle of this effect to about 0.2%, so the motor won’t move (but it might make a 25 Hz clicking sound).
- The v2 programmer would typically brown-out if a 5 V signal was applied to its RST pin while it was operating at 3.3 V. The v2.1 programmer does not have this problem.
The v2.1 programmer is otherwise identical to the v2 programmer, which means it’s a USB AVR microcontroller programmer that can program targets at 3.3 V and 5 V and offers an extra UART-type TTL serial port (like the popular FTDI USB-to-serial adapters) that can be super handy for debugging, bootloading, or even general connection of your project to a USB port.
Pololu USB AVR Programmer v2.1, labeled top view.
The v2 programmer was already a good deal at under $12, but at $7.77, and with free shipping in the USA, we hope to make AVR development extremely accessible. The manufacturing improvements and other cost reduction initiatives I have been blogging about this year help us make this offer without losing money on it, but I am not expecting to be making money directly off of the programmers, either. My goal is to give you the best value in a basic tool you will use over and over as you build your own projects, with the hope that that will help you keep Pololu in mind the next time you need some electronics or robotics parts.
And, as usual for our new product releases this year, we’re offering an extra introductory discount: the first 100 customers to use coupon code AVRPROGINTRO get that already great $7.77 price dropped to $5.55 (limit 2 per customer). (Click to add the coupon code to your cart.)
Please keep in mind this is just an issue when powering the A-Star from the programmer, so the programmer still works fine if the A-Star has its own power source, which you typically have to have, anyway. But it can be convenient, which is why we improved the feature. There are some tricks you can do, like increasing the capacitance on the programmer side or putting a small resistor in line with the power from the programmer to the A-Star. It's not something we would want to do for a production solution.
In any case, I want your reaction to be, "yay!", not "darn!" when we make better products. If you email us with your order number and reference this comment, we'll send you a couple of the new ones.
Thank you for the compliments. Yes, it possible to use this programmer to directly program AVR chips. We do not have any breadboard adapters, but you can use solid core wire or something like our premium jumper wires to make your connections.