Atmel AVR XMEGA AU series, Manual

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296
8331B–AVR–03/12
Atmel AVR XMEGA AU
Note: 1. The baud rate is defined to be the transfer rate in bits per second (bps)
For BSEL=0, all baud rates must be achieved by changing BSEL instead of setting BSCALE:
BSEL = (2
BSCALE-1
)
23.3.2 External Clock
External clock (XCK) is used in synchronous slave mode operation. The XCK clock input is sam-
pled on the peripheral clock frequency (f
PER
), and the maximum XCK clock frequency (f
XCK
)is
limited by the following:
For each high and low period, XCK clock cycles must be sampled twice by the peripheral clock.
If the XCK clock has jitter, or if the high/low period duty cycle is not 50/50, the maximum XCK
clock speed must be reduced or the peripheral clock must be increased accordingly.
Table 23-1. Equations for calculating baud rate register settings.
Operating Mode Conditions Baud Rate
(1)
Calculation BSEL Value Calculation
Asynchronous normal
speed mode (CLK2X = 0)
BSCALE
≥
0
BSCALE < 0
Asynchronous double
speed mode (CLK2X = 1)
BSCALE
≥
0
BSCALE < 0
Synchronous and master
SPI mode
f
BAUD
f
PER
16
-----------
≤
f
BAUD
f
PER
2
BSCALE
16(
⋅
BSEL 1) +
-------------------------------------------------------------- =
BSEL
f
PER
2
BSCALE
16
⋅
f
BAUD
------------------------------------------------ 1 – =
f
BAUD
f
PER
16
-----------
≤
f
BAUD
f
PER
16((2
BSCALE
BSEL
) ⋅
1) +
------------------------------------------------------------------- =
BSEL
1
2
BSCALE
---------------------
f
PER
16 f
BAUD
---------------------- 1 –
⎝ ⎠
⎛ ⎞
=
f
BAUD
f
PER
8
-----------
≤
f
BAUD
f
PER
2
BSCALE
8
BSEL
1
+
( ) ⋅ ⋅
--------------------------------------------------------------- =
BSEL
f
PER
2
BSCALE
8
⋅
f
BAUD
--------------------------------------------- 1 – =
f
BAUD
f
PER
8
-----------
≤
f
BAUD
f
PER
8((2
BSCALE
BSEL
) ⋅
1)
+
---------------------------------------------------------------- =
BSEL
1
2
BSCALE
---------------------
f
PER
8 f
BAUD
------------------- 1 –
⎝ ⎠
⎛ ⎞
=
f
BAUD
f
PER
2
-----------
<
f
BAUD
f
PER
2 BSEL 1 +
( ) ⋅
------------------------------------- = BSEL
f
PER
2
f
BAUD
-------------------
1
– =
BSCALE BSEL BSCALE BSEL
1 0
→
0 1
2 0
→
0 3
3 0
→
0 7
4 0
→
0 15
5 0
→
0 31
6 0
→
0 63
7 0
→
0 127
f
XCK
f
PER
4
-----------
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