AN2575 Freescale Semiconductor / Motorola, AN2575 Datasheet - Page 6

• Manufacturer Part Number: AN2575
• Description: MC68HC908EY16 ESCI LIN Drivers
• Manufacturer: Freescale Semiconductor / Motorola

AN2575/D
6
Example: If SCPSC = 0×73 (01110011), PDS = 3 and PSSB = 19 so FD = 4
(in decimal, SCPSC is 115 so FD = 1 +
7
PDS
0
1
2
3
7
.
.
PDS
6
PD = PDS + 1
1 (bypass)
Table 2. Fractional Divide Prescaler Register (SCPSC)
Figure 1
The input clock to the arbiter counter is selected according to the mode being
used. In the bit-time measuring mode being used here, the ACLK bit in the
arbiter control register (SCIACTL) is 0 and the arbiter counter is clocked at one-
quarter of the ESCI input clock. This input clock is selectable in CONFIG2 to
be either the bus clock or the CGMXCLK clock, which is bus × 4. In the
MC68HC908EY16 LIN drivers, the bus clock — in this case 4.9152 MHz — is
used. BPD is fixed at 2 (÷4) and BD is fixed at 1 (÷2) for LIN communications
at 9600 baud (see the technical data sheet MC68HC908EY16/D). In the
equations below, FD is the fractional divide prescaler ratio.
In two bit-times, the arbiter counter counts 2 × t
where t
The required bit-time (1÷baud rate)=64×BPD×BD×FD÷f
(see the technical data sheet MC68HC908EY16/D).
The ESCI fractional divide prescaler register (SCPSC) is programmed in two
parts as shown in
ratio of 1 to 8. (1 is a special case where the prescaler is bypassed and the
bottom five bits are ignored.) The divide ratio is one larger than the value in the
top three bits of the register, and this must be taken into account when writing
to SCPSC.
2
3
4
8
.
.
Freescale Semiconductor, Inc.
For More Information On This Product,
5
Therefore t
The bit-times are equal when 2 × count = 128 × FD;
i.e., when FD = count ÷ 64.
Bit
MC68HC908EY16 ESCI LIN Drivers
shows all the division ratios within the ESCI and its arbiter counter.
is in microseconds and f
115
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/
32
= 4
Table
Bit
4
19
= 2 × count ÷ f
/
32
2. The top three bits (PDS) constitute an integral divide
)
PSSB
31
0
1
2
3
.
.
3
PSSB (not used if PDS = 0)
Bus
Bus
is in MHz.
19
/
32
2
Bit
× f
Bus
PDFA = PSSB ÷ 32
÷ 4
Bus
1
31
1
2
3
/
/
/
0
=128×FD÷f
.
.
/
32
32
32
32
MOTOROLA
0
Bus