Until recently,  the Philips  SAA1064  LED  driver lC has  been  a sort of unofficial standard for driving seven-segment  LED  displays. lt can be used to implement  four-digit displays that can
be driven over  an l2C bus. However,  no matter how  it's packaged  (DlL24 or SO24)  this lC
is simply  on the large side with its 24 pins. Lts minimum supply voltage  of 5 V and quiescent
current  of nearly 10 mA are also not exactly state  of the  art now.

An  attractive  alternative for tasks  ofthis sort is the Maxim  MAX6958  lC. lt is available  in

the smaller QSO package  with only 16 pins, can operate at 3.3 V, and has a shutdown

mode  with a current  consumption  of only 20 pA. lnspired by this progress,  the author

resolved to design a digital thermometer  circuit using this lC. Aside from the MAX6958,

four common-cathode  LED display  modules (Toshiba TLR  324) and an Atmel  AT89C2051

microcontroller (other types could conceivably be used), all that's necessary  is a suitable temperature  sensor.  The selected  device, a National Semiconductor  lM75,  fits wellwith

the  rest ofthe electronics  because  it is also l2C compatible. The microcontroller  clock signal

for this simple  application  can  be generated by any  crystal with a frequency  in the range

of 4 to 12 MHz.

While writing the assembly  language firmware,  the author had to battle with the

complexity  of the display  driver, a consequence  ofthe  restricted  numberofpins. The

type of multiplexing  used  here by Maxim  has already  been described  in detail in Elektor  Itl.

lf you  want to know what goes  on behind  the scenes  with this driver  lC, you can  find a full

explanation in Maxim  Application  Note  1880 tzl. Naturally,  the Elektor web  page  for this

article  [:] offers  not only a ready-made  hex  file but also  the author's  fully  commented source

code  file, so you can modify  the software if you so desire.  lf you simply  want to build  the

circuit and aren't interested in programming the  microcontroller,  you can order  a pre-pro-

grammed device from the Elektor 

(080536-11)

7-8 2010  elektor

[1] Charlieplexing,  elector july & Auqust 2006

www.elektor.com/060124

[ 2] www.elektor.com/080536