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At the moment I'm working on a project with at BLE-module. This module is based on the CC2541 from TI. I want the BLE-module to permanently transmit a signal.

This module has two modes: a level- and pulse-enabled mode.

The level mode is basically using an enable pin (EN-pin). When pulled to GND the module will transmit. The problem is that the EN-pin has an internal 20k pull-up, so this means it leaks about 0.15mA (VCC = 3v) when the EN-pin is pulled to GND.

I want to prevent this current leak and want to use the pulse-enabled mode: the module will transmit for 30sec after the EN-pin is pulled to GND for >200ms, then it goes into stanby-mode. This means there is no current leak outside this 200ms pulse. I was thinking of sending a pulse every 30.2 seconds for 220ms, to "permanently" transmit a signal.

The problem is that I haven't really figured out how to send a 220ms signal every 30.2 sec. I know there are IC's around which are capable of doing it, but these are using relatively a lot of power. I'm looking for a solution in the range of a couple uA. I found the css555c, but this one is not widely available and very expensive (almost $2). I'm hoping someone can help me to find a very costs and power efficient solution.

So is there a way to send a 220ms pulse every 30.2 seconds in cost and power efficient way?

(or is there a way to prevent the current leak from the EN-pin when it's permanently tied to GND, due to the 20k pull-up)

Cheers and thanks,

R.

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  • \$\begingroup\$ I haven't read the datasheet, but it doesn't sound like there is any way around the leakage issue when en is low. How close to 30.2 s does it need to be? If the 220ms pulse comes a little early, would it be ignored (since it is already transmitting) or would it restart the 30.2 second timer? I am a little surprised because BLE is intended for low power devices (I think) and 150 uA is kind of a lot. Maybe that is why they put the pulse mode in there. \$\endgroup\$
    – mkeith
    Dec 20 '14 at 19:50
  • \$\begingroup\$ The interval should be a bit longer than 30 sec (that's why I choose 30.2, but it doesn't have to be extremely accurate).But if it sends a pulse while it's still transmitting it will stop transmitting (it acts as a toggle), so I want to prevent that since it would creating long intervals without any broadcast. \$\endgroup\$
    – Ruben
    Dec 21 '14 at 0:00
  • \$\begingroup\$ So I guess I can assume that 3V is available. Is there any clock available to use for a CMOS counter? If so, what frequency is it? \$\endgroup\$
    – mkeith
    Dec 21 '14 at 0:14
  • \$\begingroup\$ On the module (RF-BM-S02 BLE module) there is nothing available of use for a timer. Maybe on the BLE chip itself, but they didn't make the pin available. I'm thinking about making square wave generator based on a ultra low power op amp (simplecircuitdiagram.com/2010/04/25/…). I just haven't figured out how I can calculate/ influence the frequency as well as how long the pulse should be. (all the examples I found so far have a 50% duty cycle) while I want it low for 30.2 sec and high for only 200ms. \$\endgroup\$
    – Ruben
    Dec 21 '14 at 21:47
  • \$\begingroup\$ What I had in mind was feed a clock (50% duty cycle is fine) to a counter. There are several varieties of this kind of circuit. Load a value and count down, with auto-reset at 0. Let me take a look at some ideas. It may be hard to get an accurate clock without a crystal, but it may not be needed. \$\endgroup\$
    – mkeith
    Dec 22 '14 at 1:58
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You don't say which pin you mean when you say "EN pin", I assume it is simply one of the I/O pins. For the CC2541, you can determine in software whether any pin has a pull-up or not. By default they have pull-up, but you can remove that by setting the correct bit in the PxINP register.

The CC2541 also has plenty of internal timers that can be used to pull-up/down one of the I/O pins, so you don't need an external timer.

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  • \$\begingroup\$ You are right that there are indeed pins available, but I'm not using the CC2541 directly. I'm using a BLE module based on it. So I can't access all of the original pins of the CC2541 (nor firmware). And i'm also not skilled enough at this moment to do so. But I think I found an alternative way to do what I like to achieve. I will built an interrupt timer based on the TS3005 timer/oscilator (silabs.com/Support%20Documents/TechnicalDocs/TS3005.pdf) , which is fairly low cost and low power. \$\endgroup\$
    – Ruben
    Dec 29 '14 at 20:53

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