Rainbow Electronics ATmega8515L - Manual (Page 134)

134
ATmega8515(L)
2512A–AVR–04/02
Signal description:
txclk Transmitter clock. (Internal Signal)
rxclk Receiver base clock. (Internal Signal)
xcki Input from XCK pin (internal Signal). Used for synchronous slave operation.
xcko Clock output to XCK pin (Internal Signal). Used for synchronous master
operation.
fosc XTAL pin frequency (System Clock).
Internal Clock Generation –
The Baud Rate Generator Internal clock generation is used for the asynchronous and the synchronous master
modes of operation. The description in this section refers to Figure 64.
The USART Baud Rate Register (UBRR) and the down-counter connected to it function
as a programmable prescaler or baud rate generator. The down-counter, running at sys-
tem clock (fosc), is loaded with the UBRR value each time the counter has counted
down to zero or when the UBRRL Register is written. A clock is generated each time the
c o u n t e r r e a c h e s z e r o . T h i s c l o c k i s th e b a u d r a t e g e n e ra t o r c l o c k o u tp u t
(= fosc/(UBRR+1)). The transmitter divides the baud rate generator clock output by 2, 8,
or 16 depending on mode. The baud rate generator output is used directly by the
receiver’s clock and data recovery units. However, the recovery units use a state
machine that uses 2, 8, or 16 states depending on mode set by the state of the UMSEL,
U2X, and DDR_XCK bits.
Table 60 contains equations for calculating the baud rate (in bits per second) and for
calculating the UBRR value for each mode of operation using an internally generated
clock source.
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 156).
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
O SC
16 UBRR 1 +
( )
--------------------------------------- = UBRR
f
OS C
16BAUD
------------------------ 1 – =
BAUD
f
OSC
8 UBRR 1 +
( )
----------------------------------- = UBRR
f
O SC
8BAUD
-------------------- 1 – =
BAUD
f
OSC
2 UBRR 1 +
( )
----------------------------------- = UBRR
f
O SC
2BAUD
-------------------- 1 – =