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4.8 In-circuit programming of AVR
If you own an AVR programmer, you can make connections to the U3S to allow updating the firmware
without removing the AVR chip.
In this case, you should fit a 100K resistor as R3, not a jumper link. This is shown enclosed in RED on the
right side of the below diagram. The other connections required to your programmer, are MISO, MOSI,
SCK and Res (reset), these are indicated in the RED square on the diagram. The connection layout is
designed to fit a 2 x 5-pin header, to match common AVR programmer cables. Before use, you should
check that the connections match your programmer, because there are several standards in use. This
board is designed to be as flexible as possible. In some programmers the Gnd connection between MOSI
and Res is "NC" (no connection). These are also suitable for connecting to the U3S.
Notes:
1) Power needs to be applied to the U3S during programming! When you start programming, whatever
the U3S is doing at that moment will be interrupted.
2) Some constructors have reported that if the relay-switched LPF board is fitted, the relays chatter
because the programming lines are shared with relays 3 and 4, and programming errors. If this
happens with YOUR programmer, you need to either remove the relay board during programming,
or cut the wires to relays 3 and 4 and put a DPST switch in, to disconnect them when required.
3) The physical space at the 2 x 5-pin header is limited. You may not be able to fit your 2 x 5-pin
connector. Please check before trying to install it.
4.9 Higher sensitivity contrast potentiometer adjustment
The contrast voltage required by the LCD is typically under one volt, but the potentiometer R1 is connected
between +5V and Gnd. Therefore most of the available travel of the potentiometer adjustment is
redundant, and the contrast adjustment is rather sensitive. If you wish to improve this, you may fit a 220K
resistor at R4 instead of the wire jumper.
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