Actel Fusion Mixed-Signal FPGAs
Pr el iminar y v1 .7
2- 105
temperature. The post-calibration phase is enabled by bit 3 of the Mode register.
EQ 2-14 describes
the post-calibration time.
EQ 2-14
The calculation for the conversion time for the ADC is summarized in
EQ 2-15.
tconv = tsync_read + tsample + tdistrib + tpost-cal + tsync_write
EQ 2-15
tconv: conversion time
tsync_read: maximum time for a signal to synchronize with SYSCLK. For calculation purposes, the
worst case is a period of SYSCLK, tSYSCLK.
tsample: Sample time
tdistrib: Distribution time
tpost-cal: Post-calibration time
tsync_write: Maximum time for a signal to synchronize with SYSCLK. For calculation purposes, the
worst case is a period of SYSCLK, tSYSCLK.
Example
This example shows how to choose the correct settings to achieve the fastest sample time in 10-bit
mode for a system that runs at 66 MHz.
The period of SYSCLK: tSYSCLK = 1/66 MHz = 0.015 s
Choosing TVC between 1 and 33 will meet the maximum and minimum period for the ADCCLK
requirement. A higher TVC leads to a higher ADCCLK period.
The minimum TVC is chosen so that tdistrib and tpost-cal can be run faster. The period of ADCCLK
with a TVC of 1 can be computed by
EQ .
calculation will first compute the post-calibration time, second the distribution time, and finally the
STC setting.
Since Actel recommends post-calibration for temperature drift over time, post-calibration shall be
enabled and the post-calibration time, tpost-cal, can be computed by EQ 2-16. The post-calibration time is 0.24 s.
EQ 2-16
The distribution time, tdistrib, is equal to 1.2 s and can be computed using EQ 2-17. EQ 2-17
The STC value can now be computed through
EQ 2-18. The sample time is equal to 0.32 s. By
tsample = tconv – tpost-cal – tdistrib – tsync_read – tsync_write
= 1.8 s – 0.24 s – 1.2 s – 0.15 s – 0.15 s = 0.32 s
EQ 2-18
t
post-cal
MODE 3
[]
2t
ADCCLK
×
()
×
=
t
ADCCLK
4
1
TVC
+
()
×
t
SYSCLK
×
41
1
+
()
×
0.015 s
×
0.12 s
==
=
t
post-cal
2t
ADCCLK
×
0.24 s
==
t
distrib
Nt
ADCCLK
×
10
0.12
×
1.2 s
==
=
STC
t
sample
t
ADCCLK
-------------------
2
–
0.35 s
0.12 s
-------------------
2
–
2.85
==
=