Electrical Characteristics

Freescale Semiconductor

21

5.7

External Interface Timing Specifications

5.7.1

FlexBus

A multi-function external bus interface called FlexBus is provided with basic functionality to interface to slave-only devices up

to a maximum bus frequency of 66MHz. It can be directly connected to asynchronous or synchronous devices such as external

boot ROMs, flash memories, gate-array logic, or other simple target (slave) devices with little or no additional circuitry. For

asynchronous devices a simple chip-select based interface can be used.

All processor bus timings are synchronous; that is, input setup/hold and output delay are given in respect to the rising edge of

a reference clock, FB_CLK. The FB_CLK frequency may be the same as the internal system bus frequency or an integer divider

of that frequency.

The following timing numbers indicate when data will be latched or driven onto the external bus, relative to the Flexbus output

clock, FB_CLK. All other timing relationships can be derived from these values.

Conversion Time

15

—

32

cycles

Sample Time

3—

20

cycles

Multiplexer Settling Time

——

3

cycles

Zero-scale Error

ZE

—

±4

±12

lsb1

Full-scale Error

FE

—

±320

±370

lsb1

Input Capacitance

——34

pF

relates the error voltage to the observed error in conversion (code error), and is useful for systemic errors such as differential

non-linearity. A 2.56-V input on an ADC with ± 3 lsb of error could read between 0x1FD and 0x203. This unit is by far the most

common terminology and will be the preferred unit used for error representation.

A bit is a unit equal to the log (base2) of the error voltage normalized to the resolution of the ADC. An error of N bits

Table 13. FlexBus AC Timing Specifications

Num

Characteristic

Symbol

Min

Max

Unit

Notes

Frequency of Operation

—

83.33

MHz

FB1

Clock Period (FB_CLK)

12.0

—

ns

FB2

Address, Data, and Control Output Valid (FB_A[23:0], FB_D[31:0],

FB_CS[5:0], FB_R/W, FB_TS, FB_BE/BWE[3:0] and FB_OE)

—7.0

ns

1

FB3

Address, Data, and Control Output Hold (FB_A[23:0], FB_D[31:0],

FB_CS[5:0], FB_R/W, FB_TS, FB_BE/BWE[3:0], and FB_OE)

1—

ns

1, 2

Table 12. ASP Electrical Characteristics (continued)

Characteristic

Symbol

Min

Typical

Max

Unit