I/O CIRCUITS AND MODELS - Advanced Signal Integrity for High-Speed Digital Design - page 449

11.2.3 Nonlinear models

The other option for improving accuracy is to use a nonlinear model. In addi-
tion to the nonlinear current-voltage characteristic, this type of model can also
comprehend the shape of the rising and falling edges of the output. The basic
push-pull nonlinear model consists of an output current versus output voltage
curve for both pull-up and pull-down devices and an output voltage versus time
curve for both the rising and falling edges (Figure 11-9). In addition to the curves,

120

100

80

60

40

20

0

− 20

− 40

60

−

− 5 − 4 − 3 − 2 − 1 0 1 2 3

v DS ( V )

(a)

50

0

− 50

− 100

− 150

− 200

3 2 1 0 1 2 3 4 5

− − −

v DS ( V )

(b)

Figure 11-9 Nonlinear behavioral model components: (a) pull-up i out versus v out ;
(b) pull-down i out versus v out ; (c) rising edge v out versus time; (d) falling edge v out
versus time. (Continued)