at89c5131 ATMEL Corporation, at89c5131 Datasheet - Page 101

• Manufacturer Part Number: at89c5131
• Description: 8-bit Flash Microcontroller With Full Speed Usb Device At89c5131
• Manufacturer: ATMEL Corporation

Description
Figure 48. Complete Data Transfer on TWI Bus
100
AT89C5131
SDA
SCL
start
condition
S
MSB
The CPU interfaces to the TWI logic via the following four 8-bit special function regis-
ters: the Synchronous Serial Control register (SSCON; Table 85 and Table 79), the
Synchronous Serial Data register (SSDAT; Table 86), the Synchronous Serial Control
and Status register (SSCS; Table 87) and the Synchronous Serial Address register
(SSADR see Table 88 and Table 78).
SSCON is used to enable SSLC, to program the bit rate (see Table 79), to enable slave
modes, to acknowledge or not a received data, to send a START or a STOP condition
on the TWI bus, and to acknowledge a serial interrupt. A hardware reset disables SSLC.
In write mode, SSCS is used to select the TWI interface and to select the bit rate source.
In read mode, SSCS contains a status code which reflects the status of the TWI logic
and the TWI bus. The three least significant bits are always zero. The five most signifi-
cant bits contain the status code. There are 26 possible status codes. When SSCS
contains F8h, no relevant state information is available and no serial interrupt is
requested. A valid status code is available in SSCS one machine cycle after SI is set by
hardware and is still present one machine cycle after SI has been reset by software.
Table 80 to Table 83 give the status for the master modes and miscellaneous states.
SSDAT contains a byte of serial data to be transmitted or a byte which has just been
received. It is addressable while it is not in process of shifting a byte. This occurs when
TWI logic is in a defined state and the serial interrupt flag is set. Data in SSDAT remains
stable as long as SI is set. While data is being shifted out, data on the bus is simulta-
neously shifted in; SSDAT always contains the last byte present on the bus.
SSADR may be loaded with the 7-bit slave address (7 most significant bits) to which
SSLC will respond when programmed as a slave transmitter or receiver. The LSB is
used to enable general call address (00h) recognition.
Figure 48 shows how a data transfer is accomplished on the TWI bus.
The four operating modes are:
•
•
•
•
Data transfer in each mode of operation is shown in Figure 49 to Figure 52. These fig-
ures contain the following abbreviations:
S: START condition
R: Read bit (high level at SDA)
W: Write bit (low level at SDA)
1
Master Transmitter
Master Receiver
Slave Transmitter
Slave Receiver
2
7
8
signal from receiver
acknowledgement
ACK
9
while interrupts are serviced
clock line held low
1
2
3-8
ACK
signal from receiver
9
acknowledgement
condition
stop
P
4136C–USB–04/05