582 Part 3 ½ Computer Classes Section 1½ Monolithic Microcomputers
Table 1 Single-Chip Calculators
|
Chip Series |
TMS1000 |
TMS1070 |
TMS1040 |
TMS0970 |
TMS1100 |
TMS1270 |
|
Technology |
PMOS |
PMOS |
PMOS |
PMOS |
PMOS |
PMOS |
|
Number of pins per package |
28 |
28 |
28 |
28 |
28 |
40 |
|
Data operation size |
4 bits |
4 bits |
4 bits |
4 bits |
4 bits |
4 bits |
|
Instruction ROM size |
1024´ 8 bits |
1024´ 8 bits |
1024´ 8 bits |
1024´ 8 bits |
2048´ 8 bits |
1024 ´ 8 bits |
|
Data storage RAM size |
64´ 4 bits |
64 ´ 4 bits |
64´ 4 bits |
64´ 4 bits |
64´ 8 bits |
64´ 4 bits |
|
No. fixed instructions |
11 |
11 |
11 |
11 |
12 |
11 |
|
No. microprogrammable instructions |
32 |
32 |
32 |
32 |
42 |
2 |
|
Display drive |
VLED† |
VF |
VF |
VLED |
VLED† |
VF |
|
Product, function, and introduction date |
SR-16 4-function 11/74 |
TI-5100 4-function 1/76 |
2550 II 7-function 10/75 |
TI-1200 4-function 6/75 TI-1250 Little Professor |
505OM 4-function printing 6/76 5015 4-function printing |
TI-5200 4-function 1/77 |
VLED = visible-light-emitting diode
VF = vacuum flourescent
LC = liquid crystal
† Will not drive display directly; requires buffer chip.
PDP-8). Thus the ISP can be specialized to improve ROM efficiency and execution speed for a particular application.
Several special features of the microcomputer are aimed at the calculator application. The data paths are 4 bits wide to allow serial processing of binary-coded decimal (BCD) digits. The arithmetic functions assume 2's complement integers, hut special instructions can be formed to facilitate BCD arithmetic (e.g., add six to accumulator for adjustment to legal BCD digits).
Another BCD-oriented feature is the RAM addressing, which provides for four words of sixteen 4-bit fields. While the SR-16 only displays eight digits, the extras can he used as "guard" digits to maintain numerical precision or as expansion space for future products with larger displays or exponent displays. (Note that the BCD digit-serial data path allows for this expansion by increasing the loop count on variable-length-dependent operations.) The TMS 1000 applicability is primarily limited by ROM (program), RAM (user-accessible internals and temporaries), speed, and number of I/O pins.
All the calculator functions are done by program. Two time-consuming functions are display refreshing and keypad scanning. The monolithic microcomputer must serially refresh the digits of the current display value at a sufficiently high frequency to achieve "persistency." The monolithic microcomputer must also serially examine keypad rows to see if any keys have been depressed. Even simple functions, such as Add, can take several instructions to execute, since BCD digits are addressed sequentially (see the examples at the end of Chap. 34).
Due to limitations on the LSI chip density, certain architectural features add to programming complexity: