582
Automatic SCI Bit Rate Adjustment
Start
bit
Stop
bit
D0
D1
D2
D3
D4
D5
D6
D7
Low period (9 bits) measured (H'00 data)
High period
(1 or more bits)
Figure 21.9 RxD1 Input Signal when Using Automatic SCI Bit Rate Adjustment
When boot mode is initiated, the H8S/2138 or H8S/2134 Series MCU measures the low period
of the asynchronous SCI communication data (H'00) transmitted continuously from the host. The
SCI transmit/receive format should be set as follows: 8-bit data, 1 stop bit, no parity. The MCU
calculates the bit rate of the transmission from the host from the measured low period, and
transmits one H'00 byte to the host to indicate the end of bit rate adjustment. The host should
confirm that this adjustment end indication (H'00) has been received normally, and transmit one
H'55 byte to the MCU. If reception cannot be performed normally, initiate boot mode again
(reset), and repeat the above operations. Depending on the host’s transmission bit rate and the
MCU’s system clock frequency, there will be a discrepancy between the bit rates of the host and
the MCU. To ensure correct SCI operation, the host’s transfer bit rate should be set to (2400,
4800, or 9600) bps.
Table 21.7 shows typical host transfer bit rates and system clock frequencies for which
automatic adjustment of the MCU’s bit rate is possible. The boot program should be executed
within this system clock range.
Table 21.7 System Clock Frequencies for which Automatic Adjustment of H8S/2138 or
H8S/2134 Series Bit Rate is Possible
Host Bit Rate
System Clock Frequency for which Automatic Adjustment
of H8S/2138 or H8S/2134 Series Bit Rate is Possible
9600 bps
8 MHz to 20 MHz
4800 bps
4 MHz to 20 MHz
2400 bps
2 MHz to 18 MHz