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11 - Some Last Remarks
11 SOME LAST REMARKS
The ST7 has been shown at work in two different applications, where two variants have been
used.
These can be considered as the most common variants. Actually, the ST7 range is very rich,
with a wide choice of memory sizes and peripherals. This allows you to select the exact model
that will fit a given application with the best value. The choice of packages allows you to re-
duce the printed circuit area both with surface mounting and also with conventional insertion
techniques, using shrink DIP packages that reduce the component size almost by half.
The very low consumption of the ST7 has been demonstrated in the first application, where
the carrier-current receiver was powered through a capacitor and a rectifier. This is also a
useful feature in battery-powered applications, like the sailing computer described in the
second application. When this is an important feature, care should be taken to use the lowest
possible crystal frequency, since the consumption is roughly proportional to the frequency.
The ST7 has three different power-reduction features: Slow mode, Wait mode and Halt mode.
These modes offer different means of optimizing power consumption.
Halt mode is the best choice if the chip has long periods of time where it has nothing to do,
such as in applications where the controller activity is started by a human interaction, on a
keypad for example, like in pocket calculators. Restarting takes some time, so may not be us-
able where the controller must be woken-up quickly. Also, the only way of waking it is through
a reset, which is not ideal when the context must be kept from one run to the next. However,
this mode is the most efficient, since it divides the power consumption by about 1000.
Wait mode only puts the core to sleep. The peripherals are still alive, and can still handle
timing tasks, receive characters from the serial line, etc. The amount of power reduction de-
pends on the internal clock rate selected. When used in conjunction with Slow mode, the in-
ternal clock can be divided by a programmable factor (on some models), which also reduces
the consumption of the peripherals. Of course, the slow internal clock must remain compatible
with the working mode of the peripherals. This way, the consumption may be reduced from a
modest 55% to a comfortable 82%, when the ratio of 1/16 is applied to the internal clock.
The applications of described in this book have also clearly shown how powerful the 16-bit
timer is; the tasks that they have performed could not be done with a rudimentary timer even
backed up by the core. The 16-bit timer has several neat features that relieve the core from a
lot of work, even allowing it to go to sleep while pulse generation and time-keeping are still per-
formed.
The Analog to Digital converter expands the ST7 by putting it in touch with the analog world,
without using external circuitry. This can also be used to perform digital expansion, like in the
application described in the introduction where a keypad is connected to the microcontroller
using only two wires. This is typically the way satellite controls are connected to the car radios.
