Lithium battery for nrf51822

[Tizana]

hi,

I'm designing a board with nrf51822 chip. this board work on 2450 lithium rechargeable battery and it contain an MCP73831T circuit which is responsible of recharging the battery.

the MCP73831T circuit contain a STATUS output which indicate the charge STATUS. this output i want to use it as an input to the nrf51822 in order to detect the charging status level.

this output had 3 stats, low, high and high impudence.

my questions :

1/is there anyway that the nrf51822 input can detect the High Z level ? if not how can i do to read these 3 status ?

2/ the MCP73831T chip is working on a different voltage level with the nrf51822 ! the nrf51822 is powered through a step-down DC-DC converter and not directly through the battery.

so this is why i'm trying to figure out a way to connect the STATUS signal from MCP73831T to the nrf51822 .

thx,

[AmbystomaLabs]

For starters you need to reduce the STATUS output voltage to the VDD of the 51822. Anything above the 51822 VDD will cause the fets inside the nRF to turn on independent of the register states for the GPIO.

[Tizana]

i use the SPST switch circuit t o convert the voltage level : www.ti.com/.../ts5a2066.pdf

do you have any better option ? but the real question is : will the nrf circuit will be able to detect the high impedance level ?

[AmbystomaLabs]

The final solution has to be two parts. One is the output reduced to 51822 VDD to detect Hi/Lo. The other will need to be a driven solution to detect high Z.

There is no reason to use a complicated solution like the spst switch. A single fet or a zener and resistor can accomplish the voltage change.

[AmbystomaLabs]

Actually, think you can do the whole thing with 3 resistors and an ADC input on the nRF.

[AmbystomaLabs]

I think you can do something as below. You will need to go through the ohms law stuff to figure out precisely what is needed. Also, you need to look at the ADC spec to see how big C1 needs to be. But as long as the resistors are in the 500K range the leakage will be low. Basically, high Z gives 1/2 nRF VDD, Status low gives 1/3 nRF VDD and status high gives a little above 1/2 status VDD (again you need to do the ohms law stuff).