Does clock speed differs with repeater node ?

[ahmedadelhosni]

Hello,

Does a repeater node needs to run at 16Mhz rather than 8Mhz to process all packets ?
Did anyone test the two clock configurations and monitored any better configuration ?

Thanks.

[mfalkvidd]

I don't have real-world numbers but a repeater should function at almost any clock frequency.

[ahmedadelhosni]

@mfalkvidd I know it shall be tested. I depends on how much nodes do you have in your house which the repeater should handle quickly.

If someone had any lesson learned, it is great to share it.

Thanks.

[ahmedadelhosni]

Actually I was checking the Sensebender board and found that there is a place for smd crystal. I also didn't find the bypass capacitors which are connected from the crystal to the gnd. Aren't they important ?

@hek Any info about the above issue ?
Thanks.

[hek]

I haven't tried changing clock frequency to 16Mhz on the sensebender myself.
@tbowmo might have some more insight regarding this.

[ahmedadelhosni]

@hek Thanks. I will wait for a feedback.

[tbowmo]

I added a 32Khz crystal to the sensebender, as I thought it could be useful in certain circumstances. But I later decided that it wasn't needed.

The main purpose was to have a more accurate timesource for sleeps between measurements, but it doesn't matter if it measures every 60 seconds, or every 73 seconds (if the watchdog RC oscillator is running a bit slower than expected).

And for a 32Khz watch crystal, you don't need the external load capacitors., so that is the reason that they are missing.

[ahmedadelhosni]

@tbowmo Thanks for your reply.

Actually I am not familiar with what you mean by 60 or 73 seconds sleeps measurement, and the RC oscillator thing. Can you guide me what does this has to do with sleep modes please ?

I have two other questions please.

1- I can't get how will a 32Khz is good to be added ? I mean isn't the 1Mhz internal more than 32Khz ? Am I missing something ?

2- The schematic also had a pull up resistor for the CS pin. It is added to be sure that the radio is always selected but also can be neglected ?

[tbowmo]

@ahmedadelhosni

running at 32Khz oscillator (which is lower than 1Mhz) can reduce the power consumption even more. It can also be used to have an accurate timer, as a normal watch crystal is 32768Hz, which is easily divided into a 1 second clock.

When we sleep the arduino, we are using the watchdog to wake initiate an interrupt so we wake up again. This watchdog has it's own RC oscillator in the atmega. This RC oscillator is not that accurate. This means that you can not use it if you want to wake up each 60th second precisely. That is why I mention 60 and 73 seconds, as the watchdog timer might cause you to wake up every 73 seconds instead of every 60 seconds.

The schematics has a pullup on the CS pin, this is to ensure that the radio is disabled when we are downloading new firmware. @blacey had some problems downloading firmware via ISP , when doing the initial validation of the Sensebender Micro hardrware. After some trials, we found that it could be that the atmega had left the CS pin floating, and so we could (in some situations) have the radio start pulling the SPI pins on the atmega, which is also used for ISP programming.

[scalz]

completely agree too
I read some time ago an interesting article about spi bus. Maybe you already know it but I share it in case.
http://www.dorkbotpdx.org/blog/paul/better_spi_bus_design_in_3_steps

[ahmedadelhosni]

Great and clear explanation @tbowmo . Thanks a lot.

Thanks @scalz for the link. I read it and it was useful.