external oscillator and low power

[janek]

Posted on December 29, 2005 at 10:02

external oscillator and low power

[janek]

Posted on July 05, 2005 at 11:33

I have a communication application, where I need 4 serial ports and it

has to be low power, most of the time being in sleep mode. STR711FR2 looks almost very good with the exeption of the need for an external oscillator. When I checked quickly some modules for their current consumption, it is above 15-30 mA, far too high for a sleep mode requirements. Am I missing something here and/or what is the approach to manage a sleep/low power modes in case an external oscillator is a must? Does anyone know of any low power oscillators ? Jan

[hichem2]

Posted on July 07, 2005 at 06:25

Hi Janek,

Sorry for the delay... Could you tell me please, what do you mean exactly by sleep mode, it does mean (Stop mode, WFI mode, LPWFI...) And which device and peripherals feature are exactly required for your application? cheers, Hich :p

[janek]

Posted on July 08, 2005 at 00:54

Hi Hitch,

Thank you for your response. I did some work before with LPC2138 from Philips, which is OK, but it doesn't have the right peripherals for the project. The STR711FR2 looks OK, so I had a look at it and intended to get an evaluation board. Futher study however made me think twice and I'm still undecided. As I know, it is curently the only ARM7 micro, which doesn't have built in oscillator. The need for an external oscillator gives you 2 options: 1) to use a oscillator module, which is nice, but more expensive and always takes 15-30 mA 2) to build your own oscillator circuit, for which you need a crystal, a gate and a couple of RC. That comes with an extra boards space and an extra EMC problems. I am trying to understand why it is done that way and what is the best solution. There must be a reason, why ST has done it that way. Now to answer your questions. The device will be a security device powered from battery. It does nothing most of the time. Periodically it wakes up and does smth. - RTC alarms to be used. As well there are external INTs to wakeup. some of the interfaces are: serial - GSM module serial - GPS module serial - PC (including IAP) SPI - SD memory card SPI - ethernet controller ? - CMOS camera module ? - interface to host USB controller and maybe more TBD If you know of any links regarding internal versus external oscillator, please let me know. Jan

[hichem2]

Posted on July 11, 2005 at 06:06

Hi Janek,

I think that you can use the STR711FR2 in the LPWFI mode to reduce the power comsumption and in this mode you can disable the external oscillator using the a GPIO pin. I hope that this can help you Cheers, Hich :p

[huashilong]

Posted on December 26, 2005 at 07:48

Hi Janek,

You can use SG-310 from EPSON. Curren(on/off): 1.5mA/1.0uA Hi HIch, The current is 76uA in the LPWFI mode(see 10350.pdf). It is too high for many application.I want to use the STR71X in the STANDBY mode to reduce the power comsumption. I think I can enable/disable the external oscillator using the nSTDBY pin,but the matter is that STR71X will wake up before oscillator. Can I enable/disable the external oscillator using the nSTDBY pin ? Do you have another better solution? Sorry! My english is very poor.

[ppuffler]

Posted on December 29, 2005 at 10:02

Hi Janek,

you can use the LPWFI mode with connected 32kHz quartz In Sleep mode you should disable the external OscModule with a pin. and stay running on the 32kHz quartz. At the wakeup time you set the external oscilator on and switch internaly to this high freq. There is still other problem, and that is when the STR7x is in LPWFI it can be woken up by e.g. UART interrupt. Unfortunetaly after wake up are the received UART data corrupt. Paul