C8051F120/1/2/3/4/5/6/7
238
Rev. 1.2
and SDA (serial data) lines must be connected to a positive power supply voltage through a pull-up resistor or similar
circuit. Every device connected to the bus must have an open-drain or open-collector output for both the SCL and
SDA lines, so that both are pulled high when the bus is free. The maximum number of devices on the bus is limited
only by the requirement that the rise and fall times on the bus will not exceed 300 ns and 1000 ns, respectively.
20.1. Supporting Documents
It is assumed the reader is familiar with or has access to the following supporting documents:
1. The I
2
C-bus and how to use it (including specifications), Philips Semiconductor.
2. The I
2
C-Bus Specification -- Version 2.0, Philips Semiconductor.
3. System Management Bus Specification -- Version 1.1, SBS Implementers Forum.
20.2. SMBus Protocol
Two types of data transfers are possible: data transfers from a master transmitter to an addressed slave receiver
(WRITE), and data transfers from an addressed slave transmitter to a master receiver (READ). The master device ini-
tiates both types of data transfers and provides the serial clock pulses on SCL. Note: multiple master devices on the
same bus are supported. If two or more masters attempt to initiate a data transfer simultaneously, an arbitration
scheme is employed with a single master always winning the arbitration. Note that it is not necessary to specify one
device as the master in a system; any device who transmits a START and a slave address becomes the master for that
transfer.
A typical SMBus transaction consists of a START condition followed by an address byte (Bits7-1: 7-bit slave
address; Bit0: R/W direction bit), one or more bytes of data, and a STOP condition. Each byte that is received (by a
master or slave) must be acknowledged (ACK) with a low SDA during a high SCL (see Figure 20.3). If the receiving
device does not ACK, the transmitting device will read a “not acknowledge” (NACK), which is a high SDA during a
high SCL.
The direction bit (R/W) occupies the least-significant bit position of the address. The direction bit is set to logic 1 to
indicate a "READ" operation and cleared to logic 0 to indicate a "WRITE" operation.
All transactions are initiated by a master, with one or more addressed slave devices as the target. The master gener-
ates the START condition and then transmits the slave address and direction bit. If the transaction is a WRITE opera-
Figure 20.2. Typical SMBus Configuration
VDD = 5V
Master
Device
Slave
Device 1
Slave
Device 2
VDD = 3V
VDD = 5V
VDD = 3V
SDA
SCL
Summary of Contents for C8051F120
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