NEC UPD789426 Series User Manual page 222

The transmit/receive clock for the baud rate to be generated is either a signal scaled from the system clock,
or a signal scaled from the clock input to the ASCK20 pin.
(a) Generation of UART transmit/receive clock for baud rate from system clock
The transmit/receive clock is generated by scaling the system clock. The baud rate of a clock generated
from the system clock is estimated by using the following expression.
[Baud rate] =
n + 1
2
f : Main system clock oscillation frequency
X
n: Value determined by the settings of TPS200 to TPS203 as shown in Figure 12-6 (2 ≤ n ≤ 8)
Table 12-3. Example of Relationships Between System Clock and Baud Rate
Baud Rate (bps)
1,200
2,400
4,800
9,600
19,200
38,400
76,800
Caution Do not select n = 1 during operation at f
rated range.
CHAPTER 12 SERIAL INTERFACE 20
f
X
[bps]
× 8
n BRGC20 Set Value
8 70H
7 60H
6 50H
5 40H
4 30H
3 20H
2 10H
X
User's Manual U15075EJ2V1UD
f = 5.0 MHz
X
1.73
> 2.5 MHz because the resulting baud rate exceeds the
Error (%)
f = 4.9152 MHz
X
0
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