Rainbow Electronics ATmega32L - Manual (Page 141)

141
ATmega32(L)
2503C–AVR–10/02
Note: 1. The baud rate is defined to be the transfer rate in bit per second (bps).
BAUD Baud rate (in bits per second, bps)
f
OSC
System Oscillator clock frequency
UBRR Contents of the UBRRH and UBRRL Registers, (0 - 4095)
Some examples of UBRR values for some system clock frequencies are found in Table
68 (see page 163).
Double Speed Operation
(U2X) The transfer rate can be doubled by setting the U2X bit in UCSRA. Setting this bit only
has effect for the asynchronous operation. Set this bit to zero when using synchronous
operation.
Setting this bit will reduce the divisor of the baud rate divider from 16 to 8, effectively
doubling the transfer rate for asynchronous communication. Note however that the
receiver will in this case only use half the number of samples (reduced from 16 to 8) for
data sampling and clock recovery, and therefore a more accurate baud rate setting and
system clock are required when this mode is used. For the Transmitter, there are no
downsides.
External Clock External clocking is used by the synchronous slave modes of operation. The description
in this section refers to Figure 70 for details.
External clock input from the XCK pin is sampled by a synchronization register to mini-
mize the chance of meta-stability. The output from the synchronization register must
then pass through an edge detector before it can be used by the Transmitter and
receiver. This process introduces a two CPU clock period delay and therefore the maxi-
mum external XCK clock frequency is limited by the following equation:
Note that f
osc
depends on the stability of the system clock source. It is therefore recom-
mended to add some margin to avoid possible loss of data due to frequency variations.
Synchronous Clock Operation When Synchronous mode is used (UMSEL = 1), the XCK pin will be used as either clock
input (Slave) or clock output (Master). The dependency between the clock edges and
data sampling or data change is the same. The basic principle is that data input (on
RxD) is sampled at the opposite XCK clock edge of the edge the data output (TxD) is
changed.
Table 60. Equations for Calculating Baud Rate Register Setting
Operating Mode
Equation for Calculating
Baud Rate (1)
Equation for
Calculating UBRR
Value
Asynchronous Normal Mode
(U2X = 0)
Asynchronous Double Speed Mode
(U2X = 1)
Synchronous Master Mode
BAUD
f
OSC
16 UBRR 1 +
( )
--------------------------------------- = UBRR
f
OS C
16BAUD
------------------------ 1 – =
BAUD
f
O SC
8 UBRR 1 +
( )
----------------------------------- = UBRR
f
OSC
8BAUD
-------------------- 1 – =
BAUD
f
O SC
2 UBRR 1 +
( )
----------------------------------- = UBRR
f
OSC
2BAUD
-------------------- 1 – =
f
XCK
f
OSC
4
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