- 3.3V supply voltage drop because of low-cost diode D1 (see this post)
- Hard to use USB host mode, as no 5V supplied to the USB bus (see this post)
- No 5V generated from V_IN (see this post)
The good news: all of them have been improved 🙂 in the FRDM-KL25Z RevE Schematics:
The picture below shows the most important changes:
Board Power Supply
There are changes to the board with respect of power supply. The 3.3V power supply voltage drop has been fixed with better diodes with less voltage drop. Additionally the board has header J20 to bypass the voltage drop over D12:
Nearby J20 there is as well the unpopulated D7: before this diode was part of D1 on RevD. With this diode unpopulated, using the batter connector on the backside of the board requires this footprint populated (either with diode or 0-Ohm resistor).
5V Voltage Converter
As the RevD is not able to generate 5V to the Arduino header (pin 10 on J9), this makes it difficult to use it with any Arduino shield which needs to be supplied with 5V e.g. from the battery voltage. So far I have added a 5V regulator as the Pololu (seee ‘5V Power for the Shield’ in this post). Another way is to solder a 5V (e.g. Pololu tiny S7V7F5) converter to the Ardunino shield as shown in this post):
Now the board has a 3 pin header (J22) on the board so such a converter (or a 3 pin ) can be soldered.
Additionally there is space for 3 10 uC capacitors (C26, C27, C28). Alternatively a 7805 in a TO-220 package can be used as low-cost alternative.
USB Host 5V Voltage
For USB host mode, the board needs to supply 5V to the bus. So far I had to use a wire (see this post). Now the board has a header J21 and resistor R82 option:
With J21 I can supply 5V to the bus, and with R82 I can pull down the USB ID pin.
❗ The 5V are *not* protected against over-current. So I need to make sure I do not draw too much current.
The backside of the board has the RevE sticker. The pads for J6 and J8 are a bit smaller now to make it easier to solder a SWD 10pin debug connector. Additionally the wrong pin name writing has been fixed:
The new RevE fixes many limitations of the RevD board, and makes it easier to be used either in USB Host mode or with Arduino shields. Until then I’ll use my patched boards, but I’ll start using the new RevE boards in my courses as soon as I run out with the RevD boards. And I will use Mouser instead of Farnell/Element14: Farnell has increased the price (why?), and Mouser is able to deliver the boards as low as CHF 12.14 (with free shipping!) compared to Farnell/Element14 where the board costs CHF 17.05. Ordering many boards for a class makes a big difference. Definitely, the RevE board is better than RevD, and less expensive than Element14 :-).
Happy Reving 🙂