Hi guys, I want to mux two clock signals into my AT91R40008 chip because 95% of the time, it just needs to monitor the serial interface and the rest of the time it's doing some major calculations. Needless to say, two clocks = better battery life. Has anyone done this? I'm concerned when the chip first powers up, the selector for the 2:1 mux will be undefined and thus, no clock will get through, meaning the chip will never do anything... Any ideas?

Hi smcallis182,

All AT91 Family uses static design, therefore, you can stop the clock or switch between two clock sources. Regarding to the power-up period, it is requested to have valid clock signal 10 clock cycles before the NRST release because some pins are sampled for this time period like NTRI, BMS pins.

Sorry, I'm still pretty green about all this. So I can mux them in? If I just connect the mux selector pin to one of the GPIO pins on the 40008, it should select one of the clocks regardless of what state it's in, so that would get the processor started up and then I could have my code actively select which one it wants...?

Smcallis182,

It is possible to perform clock switching through PIO and multiplexing circuitry but you have to pay attention to the foolowing rules:
- you have to perform those clock switching synchronously with the targeted clock in order to avoid glitch due to the switching or no respect of the cycle in progess,
- you must have a valid clock after the reset de-assertion time,
- and of course, you have to respect rising, falling, duty cycle.... and other electrical datas given in datasheet, of course.

But I advise you to look for integrated circuit solution which embeds PLL and can manage this task correctly. It will be more easy and more sure.

Any idea what the name of such an IC would be? (Remember, I'm a n00b)

Hi smcallis182,

You can find some IC like ICS511 or other references ICS'
Take a look at and I advise to take care the following thing: your system must have a correct clock in accordance with AT91R40008 Electrical Characteristics (no overclocking or other one)

Hope this helps.

I see, I wasn't planning on overclocking it. I just wanted to have control over it to optimize my power useage. Thanks a bunch!

I've been trying to find a chip that will work. I want to have basically two speeds, something around 65 Mhz and then something much lower, such as 2-5Mhz. I can't seem to find anything that will quite work over such a range. Do you have any suggestions?

I'll have to read the datasheet on it, but it looks really good! Thanks! A bunch of the ones I was looking at had like 16 inputs, which would suck up my PIO pins like no other, a 2-wire bus will really help out. Thanks again!

This is what I use: DS1077L at www.maxim-ic.com

Simple I2C control lets you change your speed any time and you can choose to save the new speeds or re-configure on power up without saving so boot clock is the same every time!


Michael

Ugh my MCU doesn't have an I2C bus on it, so I'd have to put an external bus controller on the board which would suck up pins WORSE than the other Clock chips I was looking at. What sort of controller are you using that chip with?

AT91R40008 - just use two GPIO pins and write a software driver

I'm pretty green, how did you go about doing that? Did you just write it and store it as a function?

In the AT91 C Library v2.14 available for download on this forum, there is a software I2C example for the AT91R40008 (EB40A).

Else, I used the ICS307 from ICS to make some trials on the EB40A board.

Attached is the beta version C routines under ARM ADS and compatible with the AT91 C Library 2.14

Hope this helps.

Wow! Thanks a bunch. It's a big help. Thanks guys!