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Rev. 1.00
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Standard 8051 8-Bit Flash MCU
HT85F2260/HT85F2270/HT85F2280
UART Serial Interfaces – UART0 and UART1
UART Serial Interfaces – UART0 and UART1
UART1 Multiprocessor Communication
As UART1 can receive 9 bits in Mode A, it can be used for multiprocessor communication. When
the SM21 bit in the S1CON register is set, the receive interrupt is generated only when the 9th
received bit, the RB81 bit in the S1CON register, is high. Otherwise, no interrupt is generated upon
reception.
To utilise this feature for multiprocessor communication, the slave processors have their SM21 bit
set to high. The master processor transmits the slave’s address, with the 9th bit set high, generating
a reception interrupt in all of the slaves. The slave processors’ software compares the received byte
with their network address. If there is a match, the addressed slave clears its SM21 flag and the rest
of the message is transmitted from the master with the 9th bit set to low. The other slaves keep their
SM21 set high so that they ignore the rest of the message sent by the master. In this way, there are
reduced program overheads to distinguish the target slave MCU.
UART1 Baud Rate Setup
The UART1 operating Modes A and B, have a variable baud rate setup using the UART1 Baud
rate generator. The clock source is sourced from the system clock. The baud rate generator can be
controlled using the S1RELH and S1RELL registers and the clock is selected using the S1PRE0 and
S1PRE1 bits.
f
SYS
10-bit Time�
S1RELH[1:0]
S1RELL[7:0]
UART1
Ba�d Rate
SP1CLK
P
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f
SYS
/4
f
SYS
f
SYS
/6
f
SYS
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S1PRE0
S1PRE1
M
U
X
/
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÷16
UART1 Baud Rate Generator
The Variable baud rate can be derived using the following equation:
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1
_
_
(
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0
:
9
[
1
2
(
*
32
1
_
10
CLK
SP
of
Freq
REL
S
Rate
Baud
−
=