* Optional voltage booster on the board which can replace
the mt3608 module. If mt3608 module is used the area on
the board under the module is left unsoldered.
* Optional SPI RAM for future use to support bigger GALs/CPLDs
on the UNO board. Not currently used by any GAL.
* Support for 3.3V Arduinos by splitting the 5V power rail and
IOREF power rail. Very old Arduino UNOs R2 without IOREF
header pin can use a solder bridge JP1 (marked as Vx on the
bottom of the board) to join the 5V rail and IOREF power rail.
* HW support for internal AREF which is used to measure VPP.
There is an extra footprint on the bottom of the board
to solder another attenuation resistor (R19). For example
ESP32-S2-Mini based Arduinos with 2.5V internal analog
reference can use 100k resistor as R19 to reduce the max.
3V sensing value to 2.5V.
* ZIF18 and ZIF19 connected to the shift register output pins.
This is for supporting other GALS and CPLDs (via adapter).
* added 100K pull down resitor R18 to ZIF15 (to complement the 10k
pull-up). This is to ensure the pin is held low when the On
switch (SW1) is opened. It is used for detection the On switch
is closed when programming ATF150X via JTAG.
