Specifications

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2. High- and low-frequency crystal support with dynamic clock rate selection. The pro-
cessor’s clock rate is one of the biggest contributors to power draw. The ability to turn
off crystals and/or cores or even to slow down their s peed is cru cial in a low-power
design.
3. At least 48K of flash memory. The designers calculate that this application’s code
will fit in this memory size. Flash was selected because of its wide availability and its
ability to be reprogrammed in system. This last feature makes it the perfect selection
for non-volatile configuration storage and s upporting code that can be re-flashed with
an upgrade.
4. At least 2K of RAM. It is difficult to find an 8-bit or 16-bit MCU sup porting more
than 2K of RAM and 60K of Flash. A better solution would consist of a processor
with more RAM which still satisfies the other requirements.
5. At least f our general purpose timers to realize communication, event, and interval
timing requirements.
6. At least 60 general purpose I/O p ins with an additional 14 capable of raising externally
generated interrupts, an additional six capable of 12-bit analog-to-digital capturing,
and an additional three capable of event capture or direct compare with timers.
7. At least two Universal Synchronous Asynchronous Receiver Transmitters (USART s )
for RS-232 communication and two Serial Peripheral Interface (SPI) ports with sep-
arate pins for operation. These are necessary for interfacing to on-board memory,
external memory, RS-232 computer port, and the radio module.
8. The cost of the controller module must be less than $10.00. This cost includes the
MCU, two crystals, and supporting components.
3.2.3 SDI-12 Communications Module
The SDI-12 module is the first out of three installments that aim to satisfy the general
interface requirement. This module must meet SDI-12 V1.3 specifications [33]. Therefore,