[OpenTRV-dev] SMD version of ATmega328P

[Damon Hart-Davis]

I think we can afford at 47k pullup!

Thank you for your analysis: much appreciated.  Just hadn’t had the time to work though it properly.

Damon


On 24 Jul 2014, at 19:13, Kevin Wood <EMAIL ADDRESS HIDDEN> wrote:

> 
> 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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