LM3S9B81 Datasheet, PDF (382/1155 Pages) Texas Instruments – Stellaris® LM3S9B81 Microcontroller

External Peripheral Interface (EPI)
Register 2: EPI Main Baud Rate (EPIBAUD), offset 0x004
The system clock is used internally to the EPI Controller. The baud rate counter can be used to
divide the system clock down to control the speed on the external interface. If the mode selected
emits an external EPI clock, this register defines the EPI clock emitted. If the mode selected does
not use an EPI clock, this register controls the speed of changes on the external interface. Care
must be taken to program this register properly so that the speed of the external bus corresponds
to the speed of the external peripheral and puts acceptable current load on the pins. COUNT0 is the
bit field used in all modes except in HB8 and HB16 modes with dual chip selects when different
baud rates are selected, see page 399. If different baud rates are used, COUNT0 is associated with
the address range specified by CS0 and COUNT1 is associated with the address range specified by
CS1.
The COUNTn field is not a straight divider or count. The EPI Clock on EPI0S31 is related to the
COUNTn field and the system clock as follows:
If COUNTn = 0,
EPIClockFreq = SystemClockFreq
otherwise:
EPIClockFreq = ^{SystemClockFreq}
⎜⎛
⎝
⎢^COUNTn⎥
⎢⎣ ₂ ⎥⎦
+
1⎟⎞
⎠
×
2
where the symbol around COUNTn/2 is the floor operator, meaning the largest integer less than or
equal to COUNTn/2.
So, for example, a COUNTn of 0x0001 results in a clock rate of ½(system clock); a COUNTn of 0x0002
or 0x0003 results in a clock rate of ¼(system clock).
EPI Main Baud Rate (EPIBAUD)
Base 0x400D.0000
Offset 0x004
Type R/W, reset 0x0000.0000
31
30
29
28
27
26
25
24
23
22
21
20
19
18
17
16
COUNT1
Type RO
RO
RO
RO
RO
RO
RO
RO
RO
RO
RO
RO
RO
RO
RO
RO
Reset
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
COUNT0
Type R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Reset
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
382
June 29, 2010
Texas Instruments-Advance Information