National CP3BT26 manual Buffer Read Routine Bufflock Enabled, 122

CP3BT26

Read buffer

Read CNSTAT

Yes

RX_READY?

No

Yes

RX_BUSYx?

No

Interrupt Entry Point

RX_OVERRUN? (optional, for information)

Write RX_READY

Read buffer (id/data/control)

Read CNSTAT
		Yes
RX_BUSYx?
No
RX_FULL or		Yes
RX_OVERRUN?
No
Clear RX_PND
	Exit		DS038

that case the procedure described below must be fol- lowed.

2.Read the status to determine if a new message has overwritten the one originally received which triggered the interrupt.

3.Write RX_READY into CNSTAT.

4.Read the ID/data and object control (DLC/RTR) from the message buffer.

5.Read the buffer status again and check it is not RX_BUSYx. If it is, repeat this step until RX_BUSYx has gone away.

6.If the buffer status is RX_FULL or RX_OVERRUN, one or more messages were copied. In that case, start over with step 2.

7.If status is still RX_READY (as set by the CPU at step 2), clear interrupt pending bit and exit.

When the BUFFLOCK function is enabled (see BUFFLOCK on page 119), it is not necessary to check for new messag- es received during the read process from the buffer, as this buffer is locked after the reception of the first valid frame. A read from a locked receive buffer can be performed as shown in Figure 55.

Interrupt Entry Point

Read buffer (id/data/control)

Write RX_READY

Clear RX_PND

Exit

DS039

Figure 55. Buffer Read Routine (BUFFLOCK Enabled)

For simplicity only the applicable interrupt routine is shown:

1.Read the ID/data and object control (DLC/RTR) from the message buffer.

2.Write RX_READY into CNSTAT.

3.Clear interrupt pending bit and exit.

Figure 54. Buffer Read Routine (BUFFLOCK Disabled) 19.5.3 RX Buffer States

The first step is only applicable if polling is used to get the status of the receive buffer. It can be deleted for an interrupt driven receive routine.

1.Read the status (CNSTAT) of the receive buffer. If the status is RX_READY, no was the message received, so exit. If the status is RX_BUSY, the copy process from hidden receive buffer is not completed yet, so read CN- STAT again.

If a buffer is configured to RX_READY and its interrupt is enabled, it will generate an interrupt as soon as the buffer has received a message and entered the RX_FULL state (see also Interrupts on page 125). In

As shown in Figure 55, a receive procedure starts as soon as software has set the buffer from the RX_NOT_ACTIVE state into the RX_READY state. The status section of CN- STAT register is set from 0000b to 0010b. When a message is received, the buffer will be RX_BUSYx during the copy process from the hidden receive buffer into the message buffer. Afterwards this buffer is RX_FULL. The CPU can then read the buffer data and either reset the buffer status to RX_READY or receive a new frame before the CPU reads the buffer. In the second case, the buffer state will automat- ically change to RX_OVERRUN to indicate that at least one message was lost. During the copy process the buffer will again be RX_BUSYx for a short time, but in this case the

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