Maxim Integrated High-Speed Microcontroller Users Guide: DS80C390 Supplement - Manual (Page 85)

High-Speed Microcontroller User’s Guide: DS80C390 Supplement
85 of 158
center, EXTRQ will be set to a 1. Following the Remote Frame Request and after the
microcontroller has established the proper data to be sent, the microcontroller will
clear the TIH bit to a 0, which will allow the CAN module to send the data requested
by the previous Remote Frame Request. Note that the TIH bit associated with
Message Center 15 is ignored because it is a receive-only message center.
DTUP
Bit 0 CAN 1 Message Center 1 Data Updated. This bit indicates that new data has been
loaded into the data portion of the message center. The exact function of the DTUP
bit is dependent on whether the message center is configured in a receive (T/
R = 0)
or transmit (T/ R = 1) mode. Some functions are also dependent on the state of the
WTOE bit. The DTUP bit is only cleared by a software write to the bit, a system
reset, or the setting of the CRST bit.
T/
R =0 (receive). In this mode (T/ R = 0) the DTUP bit is set when new data has
been successfully received and is ready to be read by the microcontroller. The exact
meaning of the DTUP bit during a message center read is determined by the WTOE
bit in the CAN 1 Control Register. If WTOE = 1 (message center overwrite
enabled), DTUP should be polled before and after reading the message center to
ascertain if an overwrite of the data occurred during the read. For example, software
should clear DTUP before reading the message center and then again after the
message center read. If DTUP has been set, then a new message was received and
software should read the message center again to read the new data. If DTUP
remained cleared, no additional data was received and the data is complete. If
WTOE=0 the processor is not permitted to overwrite this message center, so it is
only necessary to clear the DTUP bit after reading the message center. The state of
the DTUP bit in the receive mode does not inhibit remote frame request transmission
in the receive mode. The only gating item for remote frame transmission in the
receive mode is that the MSRDY and MTRQ bits must both be set.
T/
R =1 (transmit). In this mode, software must set TIH =1 and clear DTUP = 0
prior to doing an update of the associated message center. This prevents the CAN
module from transmitting the data while the microcontroller is updating it. Once the
microcontroller has finished configuring the message center, software must clear
TIH = 0 and set MSRDY=MTRQ =DTUP =1, to enable the CAN module to transmit
the data. The CAN module will not clear the DTUP after the transmission, but the
microcontroller can verify that the transmission has been completed, by checking the
MTRQ bit, which will be cleared (MTRQ = 0) after the transmission has been
successfully completed.