Datasheet

ADE5166/ADE5169/ADE5566/ADE5569 Data Sheet
Rev. D | Page 138 of 156
UART2 OPERATION MODES
The UART2 has two operation modes in which each data byte
(LSB first) is preceded by a start bit (0), followed by a stop bit
(1). Therefore, each frame consists of 10 bits transmitted on the
TxD2 pin or received on the RxD2 pin.
The baud rate is set by a dedicated timer for baud rate genera-
tion, the UART2 timer, which has a fractional divisor to precisely
generate any baud rate.
Transmission is initiated by a write to the Serial Port 2 buffer SFR
(SBUF2, Address 0xEB). Next, a stop bit (1) is loaded into the
ninth bit position of the serial port shift register. The data is output
bit by bit until the stop bit appears on the TxD2 pin, and the
Serial Port 2 transmit interrupt flag, TI2 (SCON2[1]) is auto-
matically set, as shown in Figure 111.
TxD2
TI2
(SCON2[1])
START
BIT
D0 D1 D2 D3 D4 D5 D6 D7
STOP BIT
SET INTERRUPT
(FOR EXAMPLE,
READY FOR MORE DATA)
07411-188
Figure 111. 8-Bit Variable Baud Rate
Reception is initiated when a 1-to-0 transition is detected on the
RxD2 pin. Assuming that a valid start bit is detected, character
reception continues. The eight data bits are clocked into the
serial port shift register.
All of the following conditions must be met at the time the final
shift pulse is generated to receive a character:
• If the extended UART is disabled (EXTEN = 0, CFG[6]),
RI2 (SCON2[0]) must be 0 to receive a character. This ensures
that the data in the SBUF2 SFR is not overwritten if the last
received character has not been read.
• If frame error checking is enabled by setting FE2 (SCON2[4]),
the received stop bit must be set to receive a character. This
ensures that every character received comes from a valid
frame, with both a start bit and a stop bit.
If any of these conditions is not met, the received frame is irre-
trievably lost, and the Serial Port 2 receive interrupt flag, RI2
(SCON2[0]), is not set.
If the received frame meets the preceding conditions, the fol-
lowing events occur:
• The eight bits in the receive shift register are latched into
SBUF2.
• The Serial Port 2 receiver interrupt flag (RI2) is set.
Transmission is initiated by any instruction that uses SBUF2 as
a destination register. Reception is initiated by the incoming start
bit if REN2 = 1 in the SCON2 SFR, Address0xE1[2].
9-Bit UART2 with Variable Baud Rate
Setting EN-T8 (SCON2[6]) selects the 9-bit mode. In this mode,
the UART2 serial port operates in 9-bit mode with a variable baud
rate. The baud rate is set by a dedicated timer for baud rate gen-
eration, the UART2 timer, which has a fractional divisor to
precisely generate any baud rate (see the UART2 Timer Generated
Baud Rates section). The operation of the 9-bit UART2 is the
same as for the 9-bit mode of the UART.
In both modes, transmission is initiated by any instruction that
uses SBUF2 as a destination register. Reception is initiated in
8-bit mode when RI2 = 0 and REN2 = 1 in the SCON2 SFR. Recep-
tion is initiated in the 9-bit mode by the incoming start bit if
REN2 = 1.
UART2 BAUD RATE GENERATION
The baud rate is determined by the overflow rate of the dedicated
baud rate generator, the UART2 timer, which has an integer and
fractional divisor.
UART2 Timer Generated Baud Rates
The enhanced Serial Baud Rate Control 2 SFR (SBAUD2,
Address 0xEE) is used to control the UART2 timer. SBAUD2 is
the baud rate control SFR; it sets up the integer divider (DIV2,
SBAUD2[2:0]) and the extended divider (SBTH2, SBAUD2[4:3])
for the UART2 timer.
The desired value to write to DIV2 and to SBTH2 can be cal-
culated using the following formula, where f
core
is defined in the
POWCON SFR. Note that the DIV2 value must be rounded
down to the nearest integer.
DIV2 + SBTH2 =
( )
2log
16
log








× RateBaud
f
core
SBAUDF2 is the fractional divider ratio required to achieve the
required baud rate. The appropriate value for SBAUDF2 can be
calculated with the following formula:
SBAUDF2 =








−
××
×
+
1
216
64
22
RateBaud
f
SBTHDIV
core
Note that SBAUDF2 can take only two values, 0x2B or 0x07, by
clearing or by setting the SBF2 bit (SBAUD2[5]), respectively.
These values were chosen to provide an accurate baud rate for
300, 2400, 4800, 9600, 19,200, 38,400, 57,600, and 115,200 bps.
When DIV2 and SBAUDF2 are calculated, the actual baud rate
can be calculated, using the following formula:
Actual Baud Rate =






+××
+
64
2
1216
22
SBAUDF
f
SBTHDIV
core