Rainbow Electronics AT89C51SND1C - Manual (Page 162)

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AT8xC51SND1C
4109E–8051–06/03
Slave Transmitter Mode In the slave transmitter mode, a number of data Bytes are transmitted to a master
receiver (see Figure 128). Data transfer is initialized as in the slave receiver mode.
When SSADR and SSCON have been initialized, the controller waits until it is
addressed by its own slave address followed by the data direction bit which must be
logic 1 (R) for operating in the slave transmitter mode. After its own slave address and
the R bit have been received, the serial interrupt flag is set and a valid status code can
be read from SSSTA. This status code is used to vector to an interrupt service routine,
and the appropriate action to be taken for each of these status code is detailed in
Table 140. The slave transmitter mode may also be entered if arbitration is lost while the
controller is in the master mode (see state B0h).
If the SSAA bit is reset during a transfer, the controller will transmit the last Byte of the
transfer and enter state C0h or C8h. The controller is switched to the not addressed
slave mode and will ignore the master receiver if it continues the transfer. Thus the mas-
ter receiver receives all 1’s as serial data. While SSAA is reset, the controller does not
respond to its own slave address. However, the TWI bus is still monitored and address
recognition may be resumed at any time by setting SSAA. This means that the SSAA bit
may be used to temporarily isolate the controller from the TWI bus.
Miscellaneous States There are 2 SSSTA codes that do not correspond to a defined TWI hardware state (see
Table 141). These are discussed below.
Status F8h indicates that no relevant information is available because the serial interrupt
flag is not yet set. This occurs between other states and when the controller is not
involved in a serial transfer.
Status 00h indicates that a bus error has occurred during a serial transfer. A bus error is
caused when a START or a STOP condition occurs at an illegal position in the format
frame. Examples of such illegal positions are during the serial transfer of an address
Byte, a data Byte, or an acknowledge bit. When a bus error occurs, SSI is set. To
recover from a bus error, the SSSTO flag must be set and SSI must be cleared. This
causes the controller to enter the not addressed slave mode and to clear the SSSTO
flag (no other bits in S1CON are affected). The SDA and SCL lines are released and no
STOP condition is transmitted.
Note: The TWI controller interfaces to the external TWI bus via 2 port 1 pins: P1.6/SCL (serial
clock line) and P1.7/SDA (serial data line). To avoid low level asserting and conflict on
these lines when the TWI controller is enabled, the output latches of P1.6 and P1.7 must
be set to logic 1.