Download Microelectronics : Systems and Devices

Programming projects
195
Pinout
The pinout (Fig. 12.1) has been marked to show which are inputs and
which are outputs. This is done because input pins must nearly always
be connected to something. Output pins can usually be left
unconnected, at least during the early stages of development. There are
eight address input pins, which are also data output pins. These and
four of the control pins are to be connected to a Stamp2, which will be
used to command the IC. The data sheet explains that during read and
write cycles, the address is placed on the bus by the controller. At a
later stage in the cycle, data for writing is placed on the bus by the
controller, or data for reading is placed on the bus by the clock.
The data sheet has diagrams to show what other connections need to be
made to the IC. The most important is the external clock circuitry,
consisting of a crystal, and a few resistors and capacitors (Fig. 12.2).
There is a choice of frequencies. In this project we use a 32.768 kHz
crystal.
Figure 12.1 The pinout of the real time clock IC has been marked to
indicate which pins are inputs (dark grey) and which are both input
and output (light grey). The pins are also labelled with the names of
the pins they are going to be connected to on the Stamp2.