參數(shù)資料
型號: PCI1520-EP
英文描述: Military Enhanced Plastic PC Card Controllers Data Manual
中文描述: 軍事增強塑料PC卡控制器數(shù)據(jù)手冊
文件頁數(shù): 40/125頁
文件大小: 716K
代理商: PCI1520-EP
3
12
in Figure 3
9. The end of a requested data transfer is indicated by a stop condition, which is signaled by a low-to-high
transition of SDA while SCL is in the high state, as shown in Figure 3
9. Data on SDA must remain stable during the
high state of the SCL signal, as changes on the SDA signal during the high state of SCL are interpreted as control
signals, that is, a start or a stop condition.
SDA
SCL
Start
Condition
Stop
Condition
Change of
Data Allowed
Data Line Stable,
Data Valid
Figure 3
9. Serial-Bus Start/Stop Conditions and Bit Transfers
Data is transferred serially in 8-bit bytes. The number of bytes that may be transmitted during a data transfer is
unlimited; however, each byte must be completed with an acknowledge bit. An acknowledge (ACK) is indicated by
the receiver pulling the SDA signal low, so that it remains low during the high state of the SCL signal. Figure 3
10
illustrates the acknowledge protocol.
SCL From
Master
1
2
3
7
8
9
SDA Output
By Transmitter
SDA Output
By Receiver
Figure 3
10. Serial-Bus Protocol Acknowledge
The PCI1520 is a serial bus master; all other devices connected to the serial bus external to the PCI1520 are slave
devices. As the bus master, the PCI1520 drives the SCL clock at nearly 100 kHz during bus cycles and places SCL
in a high-impedance state (zero frequency) during idle states.
Typically, the PCI1520 masters byte reads and byte writes under software control. Doubleword reads are performed
by the serial EEPROM initialization circuitry upon a PCI reset and may not be generated under software control. See
Section 3.6.3,
Serial-Bus EEPROM Application
, for details on how the PCI1520 automatically loads the subsystem
identification and other register defaults through a serial-bus EEPROM.
Figure 3
11 illustrates a byte write. The PCI1520 issues a start condition and sends the 7-bit slave device address
and the command bit zero. A 0 in the R/W command bit indicates that the data transfer is a write. The slave device
acknowledges if it recognizes the address. If no acknowledgment is received by the PCI1520, then an appropriate
status bit is set in the serial-bus control and status register (PCI offset B3h, see Section 4.48). The word address byte
is then sent by the PCI1520, and another slave acknowledgment is expected. Then the PCI1520 delivers the data
byte MSB first and expects a final acknowledgment before issuing the stop condition.
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