Freescale Semiconductor MPC5604B, Quick Start Manual

Page: 102 / 150

Standard-
mode
Fast-mode Fast-mode plus
Maximum Bit Rate( 𝑓
𝑆𝐶𝐿
) 100kbps 400kbps 1000kbps
𝑉
𝑂𝐿𝑚𝑎𝑥
Maximum low level
output
0.4V 0.4V/0.6V 0.4V
𝐼
𝑂𝐿
Low level output current 3mA 3mA/6mA 20mA
𝑡
𝑟𝑚𝑎𝑥
Maximum rise time 1000ns 300ns 120ns
𝑡
𝑓𝑚𝑎𝑥
Maximum fall time 300ns 300ns 120ns
𝐶
𝑏𝑢𝑠
𝑚𝑎𝑥
Bus capacitance 400pF 400pF 550pF
𝑡
𝐻𝑂𝐿𝐷 _ 𝑆𝑇𝐴𝑅𝑇
Minimum time between
start signal and clock start
4.7µs 0.6µs 0.26µs
𝑡
𝐻𝑂𝐿𝐷 _ 𝑆𝑇𝑂𝑃
Min. time between clock
release and stop signal
4.7µs 0.6µs 0.26µs
𝑡
𝑉𝐷
𝑆𝐷𝐴
Max. time for valid SDA
after SCL falling edge
3.45µs 0.9µs 0.45µs
0.6V/6mA situation in fast-mode is the worst case situation where 𝐶
𝑏𝑢𝑠
𝑚𝑎𝑥
= 400 𝑝𝐹
and 𝑓
𝑆𝐶𝐿
= 400 𝑘𝐻𝑧 .
Devices and the cable sizes create parasitic capacitances that add up to be the bus capacitance
𝐶
𝑏𝑢𝑠
. For an accurate value, this has to be measured someway, but if it is not possible, a good
way of estimating it is 𝐶
𝑏𝑢𝑠
= 𝐶
𝑑𝑒𝑣𝑖𝑐𝑒
+ 𝐶
𝑤𝑖𝑟𝑒
, where 𝐶
𝑑𝑒𝑣𝑖𝑐𝑒
is 20pF for each device pad (10pF
for pad load+10pF for connections) and 𝐶
𝑤𝑖𝑟𝑒
is estimated on cable by assuming 50-100pF/m
on cables.
1.3. Pull-up resistor calculation
Pull-up resistance value is limited by low level current consumption of the devices and the RC
time constant of the bus cause by the bus capacitance.
The bus capacitance limitation can be calculated using RC charging formula. In I²C specifications,
input Low and High values are defined respectively as 30% and 70% of the supply Vdd. By
solving
𝑣 ( 𝑡 ) = 𝑉
𝑑𝑑
� 1 − 𝑒
−𝑡 / 𝑅𝐶
� for 𝑡 at 0.3 at 0.7 values, we get 𝑡
30%
= 0.356675 × 𝑅𝐶 and
𝑡
70%
= 1.203970 × 𝑅𝐶 , and then 𝑡
𝑟
= 0.847298 × 𝑅𝐶 .
So the maximum pull-up resistance value is: 𝑅
𝑝
𝑚𝑎𝑥
=
𝑡
𝑟𝑚𝑎𝑥
0 . 847298 × 𝐶
𝑏𝑢𝑠
.
The minimum value is limited by the current consumption of the device, using ohm’s law when
the output of a device is at low, we have
𝑅
𝑝
𝑚𝑖𝑛
=
𝑉
𝑑𝑑
−𝑉
𝑂𝐿
𝐼
𝑂𝐿
.
Example at 100pF and with a baud rate of 400kHz:
𝑅
𝑝
𝑚𝑎𝑥
=
300 × 10
−9
0.847298 × 100 × 10
−12
= 3541 Ω ; 𝑅
𝑝
𝑚𝑖𝑛
=
5 − 0.4
3 × 10
−3
= 1533 Ω
Therefore a pull-up resistor of 2.2k Ω is a good choice at this specifications.
2. Using the I²C module
2.1. Module Presentation
There is only one I²C module in MPC5604B and it features pretty much every I²C features except
for general call address and 10-bit addressing. These modules pins and signals are compatible