68HC11 Learning tools

[Wen Yen Chan]

On Mon, 20 Jan 1997 FJB203 at aol.com wrote:

> In a message dated 97-01-20 00:43:08 EST, you write:
> 
> > Another great product is Dallas's Smart Socket.  It provides battery backup
> >  for any low power CMOS RAM just by plugging it into the socket.  Also
> 
> YOu can also do that yourself by putting a litium 4.5v battery in series with
> a diode to the memory chip, thus preventing the battery from being charged,
> and most ram chips can't be accessed when the voltage is about 4v or less
> (not all, but most), which the diode also does (most small signal diodes have
> a voltage drop of about .6 volts.
> 
> Fred
> 


I have used both the smart socket and the simple diode schemes to backup
low power SRAM chips. While a single diode is much cheaper than a
smartSocket it does have a distinct disadvantage. When the power is killed
the smart socket can detect an impending low voltage (on the power rail)
condition and prevent random writes to the sram by pulling !chipSelect &
!writeEnable high. If the simple diode scheme must be used for cost
reasons I would add a large cap, several pull up resistors
(for the control lines) and replace the diode with a hot carrier diode
(0.25v forward drop). 

Wen