Fujitsu MB90390 Series Hardware Manual page 405

■ Using External Clock
If the EXT bit of the SMR2/SMR3 is set, an external clock is selected, which has to be connected to the
SCK2/SCK3 pin. The external clock is used in the same way as the machine clock to the baud rate reload
counter.
If One-to-one External Clock Input Mode (SMR2/SMR3:OTO) is selected the SCK2/SCK3 signal is
directly connected to the UART2, UART3 serial clock inputs. This is needed for the UART2, UART3
synchronous mode 2 operating as slave device.
Note, that in any case the resulting clock signal is synchronized to the machine clock in the UART2,
UART3 module. This means that indivisible clock rates will result in phase unstable signals.
■ Counting Example
Assume the reload value is 832. The Figure 20.6-2 demonstrates the behavior of both Reload Counters.
Transmission/
Reception Clock
Reload
001
Count
Transmission/
Reception Clock
Reload
418
Count
Note:
The falling edge of the Serial Clock Signal always occurs | (v + 1) / 2 | machine clock cycles after the
rising edge.
Figure 20.6-2 Counting Example of the Reload Counters
000 832 831
reload count value
417 416
830 829
415 414 413
CHAPTER 20 UART2, UART3
828 827
412 411
377