Page 244 - The Art of Designing Embedded Systems

A Simple Drawing System 231

The ROM or PAL BOM will be called out with quantity of 0. This
procedure really violates the definition of the drawing system, but it dras-
tically reduces the number of drawings needed by production to build a
unit.
On the PC board BOM, the callout for a ROM or PAL will look like:
Item Qty Part # Description Ref

I 1 GAL22V10 PAL U19
2 0 #1234-1 (MASTERSU’RODAW-Ul9.PDS) B9

Thus, the first entry tells us what to buy and where to put it; the sec-
ond refers to engineering documentation and the current checksum. For a
ROM, list the version number instead of the checksum. The description
field for the part must also include the ROM or PAL’S file name in paren-
theses, with directory on the lab computer.
ROMs, PALS, and SLD will be defined via BOMs, since these ele-
ments are really composed of potentially numerous sets of documentation.
The ROM/PAL/SLD drawing will form the basic linkage to all source
code files used in their creation.
The primary component of a PALEOM drawing is of course the de-
vice itself. Other rows will list the files needed to build the ROM or PAL.
Where two ROMs are derived from one set of code (like EVEN and
ODD ROMs), these will both be on the same drawing.
An example ROM follows:
Item Qty Part# Description Ref

-1
1 1234- 1 64 1 80 P-bd ROM U9
1 27256- 10 EPROM, 100 nsec
2 PRODA.MAK-make file proda\code

Note that in this part list the EPROM itself is called out by conven-
tional part number, but the quantity is 0 (since a quantity was called out on
the PC board BOM that referenced this drawing).
A ROM, PAL, or SLD drawing calls out the ingredients of the de-
vice. In this case, the software’s MAKE is listed so there’s a reference
from the hardware design to the firmware configuration.
If other engineering documentation exists, it should be referred to as
well. This could include code descriptions, etc.
The last column contains the directory where these things are stored
on the network drive.