AT7908E
The
error_counters
block is a counter that receives signals from
can_tx
and
can_rx
to
calculate the error state of the CAN. If the number of errors is higher than 127, the
AT7908E will be on the error passive state. If error counters is higher than 255, the
AT7908E will be on the bus-off state.
The
synchronizer
block synchronizes the controller with the
can_rx
incoming signal
and generates the internal clock and reset signals for the other blocks of the controller.
The stuff handler block handles the stuff bit generation, compliant with the standard
BOSCH 2.0B specification.
error_frame_gen
block generates the error frame on the
bus.
In the next sections, the interface block, the internal registers and the operational modes
of the AT7908E will be described.
Transmission of Message
To transmit a message on the bus, the controller must program the configuration regis-
ters of the AT7908E (interrupt generator configuration, data rate, bit timing
configuration, message length) and the transmit buffer. After this, the controller must
write in the internal register the bit (Txreq=1) to start the transmission. At this point, the
Transmit message buffer is sent to the bus line. At every moment the external MCU can
readout the status of the controller (error condition, bus-off condition, transmission
active or transmission OK).
The incoming message passes through the global mask and is checked with the identi-
fier of the first receiver message buffer. If the message is accepted, the data will be
stored in the first message buffer and the status of the receiver message objects will be
updated with the information of the received message: length of message, extended or
standard frame message, reception OK or overrun condition. The incoming message is
stored in the message buffer at the end of the END of FRAME field (7 recessive bits).
If the incoming message is a remote frame request from another CAN node, the incom-
ing remote request will not be stored on the RX message buffer.
Remote Frame
The AT7908E supports remote frame features. To send on the bus a remote frame
request, the MCU must write two internal bits of SETUP register (TMRMR=1 and
TXRM=0). The arbitration registers of the transmitter message buffer must have the
identifier for the remote frame request. The start of the remote frame request will be per-
formed when the transmission request is set by the MCU on the AT7908E. The
AT7908E that receives a REMOTE FRAME request (RTR bit =recessive) that matches
the identifier stored on own TX_arbitration register (filtering functionality with don’t care
feature) send on the bus the TX message buffer as an answer to the remote frame
request. To answer the remote frame request, the AT7908E must be pre-programmed
with two bits of the internal register: TMRMR=0 and TXRM=1. The TX arbitration regis-
ter must be initialized with the message identifier to which to answer and the transmitter
data buffer must be loaded with the data to be sent as the answer to the remote frame
request. The transmission request must be set as:
Answer to remote request if
for i = 28 down to 18
(“MsgID(i)” XOR” TX_arbitration (i) )AND “Accept. Mask(i)” = 0
Filtering of Message
The filtering function is implemented with a global mask and the arbitration register of
the receiver message buffers. The Global Mask permits to define the don’t care bit on
the filtering operation. The incoming message ID (msg_ID(28:18) is checked (xor) with
Incoming Message
7
4268D–AERO–11/09
ATMEL [ ATMEL CORPORATION ]
