1 threefold bit sampling -66, 2 bus rate timing example -66 – Maxim Integrated MAXQ7666 User Manual
Page 196
MAXQ7665/MAXQ7666 User’s Guide
4-66
The following restrictions apply to the above equations:
The nominal bit time applies when a synchronization edge falls within the t
SYNC_SEG
period. The maximum bit time occurs when the
synchronization edge falls outside of the t
SYNC_SEG
period, and the synchronization jump width time is added to perform the resyn-
chronization.
4.15.1 Threefold Bit Sampling
The MAXQ7665/MAXQ7666 support the ability to perform one or three samplings of each bit, based on the SMP bit (C0BT1.7). The
single sample mode (SMP = 0) is available in all settings and takes one sample during each bit time. The triple sampling mode
(SMP = 1) samples each bit three times for increased noise immunity. This mode can only be used when the baud-rate prescale value
(BPRV) is greater than 3.
4.15.2 Bus Rate Timing Example
Table 4-5 shows an example bit timing setting for an 8MHz oscillator frequency and some baud-rate selections. Because of the large
number of variables, there are many combinations not shown that can achieve a desired baud rate. There are a number of approach-
es to determining all the bit timing factors, but this uses the most common (i.e., the oscillator frequency and baud rate have already
been determined by system constraints).
Table 4-5. Bit Timing Setting Example for 8MHz Oscillator Frequency
To understand the table data, the following is an explanation of how the 8MHz oscillator frequency and a 125kbps CAN baud rate data
is derived.
Various combinations of BRPV are selected until one is located that meets the "t
QU
per bit" criteria, i.e., an integer value less than 25.
Selecting BRPV = 2, the previously described equations state that there should be 16 t
QU
per bit. That leaves 16-1 or 15 t
QU
remain-
ing for TS1_LEN and TS2_LEN, which are arbitrarily assigned as shown. Because BRPV < 3, the triple sampling feature (SMP = 1) can-
not be used.
no
al bit time
t
t
t
BRPV CCD
TS
LEN
TS
LEN
f
imum bit time
t
t
t
t
BRPV CCD
TS
LEN
TS
LEN
SJW
f
CAN baud rate
f
BRPV CCD
SYNC SEG
TSEG
TSEG
OSC
SYNC SEG
TSEG
TSEG
SJW
OSC
OSC
min
_
_
max
_
_
_
_
=
+
+
=
(
)(
)
+
(
)
+
(
)
[
]
=
+
+
+
=
(
)(
)
+
(
)
+
(
)
+
(
)
[
]
=
(
)
1
2
1
2
1
1
2
1
1
2
((
)
+
(
)
+
(
)
[
]
1
1
2
TS
LEN
TS
LEN
_
_
t
t
t
t
t
t
TS
LEN
TS
LEN
TS
LEN TS
LEN
TSEG
TSEG
TSEG
SJW
SJW
TSEG
1
2
2
1
2
1
16
2
2
8
1
2
1
25
≥
≥
<
≤
≤
≤
≤
+
+ ≤
_
_
(
_
_
)
f
OSC
CCD
BRPV
t
QU
BAUD RATE
t
QU
PER
BIT
TS1_LEN
TS2_LEN
SJW
SMP = 1
PERMITTED?
1
1
125ns
1Mbps
8
4
3
2
No
1
1
125ns
500kbps
16
10
5
4
No
1
1
250ns
250kbps
16
10
5
4
No
8MHz
2
2
500ns
125kbps
16
10
5
4
No
Maxim Integrated