Rainbow Electronics MAX5812 - Manual (Page 9)

MAX5812
12-Bit Low-Power, 2-Wire, Serial
Voltage-Output DAC
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low transition on SDA with SCL high. A STOP condition
is a low-to-high transition on SDA while SCL is high
(Figure 2). A START condition from the master signals
the beginning of a transmission to the MAX5812. The
master terminates transmission by issuing a not
acknowledge followed by a STOP condition (see the
Acknowledge Bit section). The STOP condition frees the
bus. If a repeated START condition (S
r
) is generated
instead of a STOP condition, the bus remains active.
When a STOP condition or incorrect address is detect-
ed, the MAX5812 internally disconnects SCL from the
serial interface until the next START condition, minimiz-
ing digital noise and feedthrough.
Early STOP Conditions
The MAX5812 recognizes a STOP condition at any point
during transmission except when a STOP condition
occurs in the same high pulse as a START condition
(Figure 3). This condition is not a legal I
2 C format, at
least one clock pulse must separate any START and
STOP conditions.
Repeated START Conditions
A repeated start (S r ) condition might indicate a change
of data direction on the bus. Such a change occurs
when a command word is required to initiate a read
operation. S
r also can be used when the bus master is
writing to several I 2 C devices and does not want to
relinquish control of the bus. The MAX5812 serial inter-
face supports continuous write operations with or with-
out an S
r condition separating them. Continuous read
operations require S r conditions because of the change
in direction of data flow.
Acknowledge Bit (ACK)
The acknowledge bit (ACK) is the ninth bit attached to
any 8-bit data word. ACK is always generated by the
receiving device. The MAX5812 generates an ACK
when receiving an address or data by pulling SDA low
during the ninth clock period. When transmitting data,
the MAX5812 waits for the receiving device to generate
an ACK. Monitoring ACK allows detection of unsuc-
cessful data transfers. An unsuccessful data transfer
occurs if a receiving device is busy or if a system fault
has occurred. In the event of an unsuccessful data
transfer, the bus master should reattempt communica-
tion at a later time.
Slave Address
A bus master initiates communication with a slave
device by issuing a START condition followed by the
7-bit slave address (Figure 4). When idle, the MAX5812
waits for a START condition followed by its slave
address. The serial interface compares each address
value bit-by-bit, allowing the interface to power-down
immediately when an incorrect address is detected.
The LSB of the address word is the Read/Write (R/W)
bit. R/W indicates whether the master is writing to or
reading from the MAX5812 (R/W = 0 selects the write
condition, R/W = 1 selects the read condition). After
receiving the proper address, the MAX5812 issues an
ACK by pulling SDA low for one clock cycle.
The MAX5812 has eight factory/user-programmed
addresses (Table 2). Address bits A6 through A1 are
preset; A0 is controlled by ADD. Connecting ADD to
GND sets A0 = 0. Connecting ADD to V
DD sets A0 = 1.
This feature allows up to eight MAX5812s to share a bus.
Write Data Format
In write mode (R/W = 0), data that follows the address
byte controls the MAX5812 (Figure 5). Bits C3–C0 con-
figure the MAX5812 (Table 3). Bits D11–D0 are DAC
data. Input and DAC registers update on the falling
edge of SCL during the acknowledge bit. Should the
write cycle be prematurely aborted, data will not be
updated and the write cycle must be repeated. Figure
6 shows two example write data sequences.
S A6 A5 A4 A3 A2 A1 A0 R/W
Figure 4. Slave Address Byte Definition
Table 2. MAX5812 I 2 C Slave Addresses
PART V ADD
DEVICE ADDRESS
(A 6 ...A 0 )
MAX5812L GND 0010 000
MAX5812L V DD 0010 001
MAX5812M GND 0010 010
MAX5812M V DD 0010 011
MAX5812N GND 0110 100
MAX5812N V DD 0110 101
MAX5812P GND 1010 100
MAX5812P V DD 1010 101
C3 C2 C1 C0 D11 D10 D9 D8
Figure 5. Command Byte Definition