Freescale Semiconductor MPC5200B User Manual page 584

Granularity
Level
(value multiply by 4)
(example: 0x005)
Alarm Level
"almost Empty"
(example: 0x010)
15.4.1 RX FIFO
The RX FIFO space is 512 Byte. For an Rx FIFO, the "Alarm" value is not the amount of "data" in the Rx FIFO. Instead, an interrupt occurs
as a result of the amount of empty space remaining in the Rx FIFO. These facts are described in
If it is known how much data is needed in the Rx FIFO to cause an interrupt, the value that must be written into the "Alarm" register is:
• the FIFO size, minus the number of data bytes in the FIFO
Unlike the "Alarm" value, "Granularity" value represents a number of data bytes, not empty space.
In AC97, the number of data bytes are 4-times the number of time slot samples in the FIFO. Because,
each 20-bit sample uses an entire 32-bit longword in the FIFO.
For the Rx FIFO, the value can be between 0 and 7 bytes only. Therefore, the interrupt has hysteresis. For example, the interrupt goes active
when the Rx FIFO is "almost full" (i.e., amount of empty space is less than the "Alarm" level). It stays active until enough data is read out of
the Rx FIFO so that the amount of data left in the FIFO is less than the "Granularity" level.
For the example (see Figure 15-22)
The requestor to the BestComm to emptying the RX FIFO becomes active if the empty space in the FIFO is less the 8 Bytes (504 date
Bytes are in the FIFO).
The requester became inactive if 4Bytes are left in the FIFO. (508 Byte space now)
When BestComm is servicing the FIFO's, this process works well. However, if the CPU is servicing the FIFO's, the interrupt has no hysteresis.
For Example, the "Alarm" level is used for both activating and deactivating the CPU interrupt.
Freescale Semiconductor
CommBus or IPB
Interface
Address:
1FF
empty FIFO
Space
last Byte to send
0
first Byte to send
Transmitter
Tx Line
Figure 15-22. PSC FIFO System
it means:
MPC5200B Users Guide, Rev. 1
0
first received Byte
last received Byte
empty FIFO
Space
1FF
Receiver
Rx Line
Figure
NOTE
PSC FIFO System
Granularity
Level
(example: 0x004)
Alarm Level
"almost Full"
(example: 0x008)
15-22.
15-73