Intel 87C196CB, 8XC196NT user manual Programming the Message Object Control Registers, Access Type

CAN SERIAL COMMUNICATIONS CONTROLLER

7.5.3Programming the Message Object Control Registers

Each message object control register consists of four bit pairs — one bit of each pair is in true form and one is in complement form. This format allows software to set or clear any bit with a single write operation, without affecting the remaining bits. Table 7-12 shows how to interpret the bit-pair values.

Table 7-12. Control Register Bit-pair Interpretation

Access Type	MSB	LSB	Definition
	0	0	Not allowed (indeterminate)
Write	0	1	Clear (0)
	1	0	Set (1)
	1	1	No change
Read	0	1	Clear (0)
	1	0	Set (1)

7.5.3.1Message Object Control Register 0

Message object control register 0 (Figure 7-14) indicates whether an interrupt is pending, controls whether a successful transmission or reception generates an interrupt, and indicates whether a message object is ready to transmit.

7.5.3.2Message Object Control Register 1

Message object control register 1 (Figure 7-15) indicates whether the message object contains new data, whether a message has been overwritten, whether the message is being updated, and whether a transmission or reception is pending. Message objects 1–14 have only a single buffer, so if a second message is received before the CPU reads the first, the first message is overwritten. Message object 15 has two alternating buffers, so it can receive a second message while the first is being processed. However, if a third message is received while the CPU is reading the first, the second message is overwritten.

7.5.4Programming the Message Object Data

Each message object can have from zero to eight bytes of data. For transmit message objects, write the message data to the data registers (Figure 7-16). For receive message objects, the CAN controller stores the received data in these registers. The CAN controller writes random values to any unused data bytes during operation, so you should not use unused data bytes as scratch-pad memory.

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