[OpenTRV-dev] SMD version of ATmega328P

[Kevin Wood]

 From Atmel's ISP data sheet (AVR910):


To avoid driver contention, a series resistor should be placed on each 
of the three dedicated lines if there is a possibility that exter-
nal circuitry could be driving these lines. The connection is shown in 
Figure 3. The value of the resistors should be chosen depending on the 
circuitry connected to the SPI bus.

Note that the AVR microcontroller will automatically set all its I/O 
pins to inputs, with pull ups disabled, when Reset is active.

So, we just need to ensure the RFM23B doesn't go active during 
programming. A pullup on the nSel line or (better) a connection to the 
RESET line, perhaps, that holds the RFM23 nSel pin high when RESET is 
low would do it. Maybe we can rely on it to float high?

Kevin


On 24/07/14 17:31, Damon Hart-Davis wrote:
> Hi,
>
> We’re starting production engineering work in earnest very soon, and we’re going to need to support in-circuit programming to use SMD.
>
> I believe the standard way to do this involves driving the SPI bus as for loading a bootloader.
>
> I am concerned that this cannot be done without damaging the RFM23B attached to the SPI bus, or am I wrong about that?  What gotchas don’t I even know about yet?
>
> Thanks
>
> Damon
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