Specifications

as to the stability of the Spansion part. So both chips were included in layout to provide

redundancy.

4.1.9 Removable Memory

Secure Digital [47] (SD) and Compact Flash [48] (CF) are two of the most popular and

widely available forms of removable ﬂash memory. The other considerations for removab le

ﬂash memory were rejected because any implementation detail requires fees to be paid to

the individual ﬂash memory institution. CF cards use a total of 60 pins to interface to a

host. Technically, only 20 are needed to interface in its smallest supported mode. It can

appear to operate as an IDE disk drive, memory-mapped storage, or I/O-mapped ports. A

big drawback in its basic operations is th at is uses parallel address and data busses. This is

very fast, but the MCU cannot support this interface. The SD interface has nine pins and

supports ﬁve mod es: SD0-SD3 and SPI. The SD0-SD3 modes support a 1- to 4-bit parallel

interface. The SPI mode is compliant with the SPI communications module supported by

the MCU. This is a three wire interface and all card features are still fully accessible.

The SD card was chosen as the preferred type of removable memory. L ocal stores

currently carry 512MB or 1GB for about $15. For some perspective, 1MB is enough m emory

to hold three months worth of data generated by a device with six sensors sampled every

15 minutes. At this rate, 512MB could record data from an entire network of 32 loggers for

one year.

4.1.10 User Interface Module

Two types of screens were considered for use one this project: a cu s tom LCD display,

or a LCD character display. Acclima has had experience in the p ast with making their own

custom glass. Figure 4.4 shows the ﬁr s t attempts at the artwork for the custom L CD. All

black symbols repr esent segments that would be activated individually. T his screen would

need a 128-segment LCD controller. Power wise, custom glass would only use 1-2mA if all

the segments were active at the same time. The drawback to this method is th at changes,

upgrades, or supporting other languages would be expensive and units already released