Automatic Alignment of SCI Bit Rate
Start
bit
Stop
bit
D0
D1
D2
D3
D4
D5
D6
D7
This low period (9 bits) is measured (H'00 data)
High for at
least 1 bit
Figure 19.5 Measurement of Low Period in Data Transmitted from Host
When started in boot mode, the H8/3334YF measures the low period in asynchronous SCI data
transmitted from the host (figure 19.5). The data format is eight data bits, one stop bit, and no
parity bit. From the measured low period (9 bits), the H8/3334YF computes the host’s bit rate.
After aligning its own bit rate, the H8/3334YF sends the host 1 byte of H'00 data to indicate that
bit-rate alignment is completed. The host should check that this alignment-completed indication is
received normally and send one byte of H'55 back to the H8/3334YF. If the alignment-completed
indication is not received normally, the H8/3334YF should be reset, then restarted in boot mode to
measure the low period again. There may be some alignment error between the host’s and
H8/3334YF’s bit rates, depending on the host’s bit rate and the H8/3334YF’s system clock
frequency. To have the SCI operate normally, set the host’s bit rate to 2400, 4800, or 9600 bps*1.
Table 19.8 lists typical host bit rates and indicates the clock-frequency ranges over which the
H8/3334YF can align its bit rate automatically. Boot mode should be used within these frequency
ranges*2.
Table 19.8 System Clock Frequencies Permitting Automatic Bit-Rate Alignment by
H8/3334YF
System Clock Frequencies Permitting Automatic Bit-Rate Alignment
by H8/3334YF
Host Bit Rate*1
9600 bps
8 MHz to 16 MHz
4 MHz to 16 MHz
2 MHz to 16 MHz
4800 bps
2400 bps
Notes: *1 Use a host bit rate setting of 2400, 4800, or 9600 bps only. No other setting should be
used.
*2 Although the H8/3334YF may also perform automatic bit-rate alignment with bit rate
and system clock combinations other than those shown in table 19.8, there will be a
slight difference between the bit rates of the host and the H8/3334YF, and subsequent
transfer will not be performed normally. Therefore, only a combination of bit rate and
system clock frequency within one of the ranges shown in table 19.8 can be used for
boot mode execution.
386
RENESAS [ RENESAS TECHNOLOGY CORP ]
