LAN9117 Datasheet, PDF (95/114 Pages) SMSC Corporation – HIGH PERFORMANCE SINGLE-CHIP 10/100 NON-PCI ETHERNET CONTROLLER

LAN9117
6.0 TIMING DIAGRAMS
6.1 Host Interface Timing
The LAN9117 supports the following host cycles:
Read Cycles:
• PIO Reads (nCS or nRD controlled)
• PIO Burst Reads (nCS or nRD controlled)
• RX Data FIFO Direct PIO Reads (nCS or nRD controlled)
• RX Data FIFO Direct PIO Burst Reads (nCS or nRD controlled)
Write Cycles:
• PIO writes (nCS and nWR controlled)
• TX Data FIFO direct PIO writes (nCS or nWR controlled)
6.1.1 SPECIAL RESTRICTIONS ON BACK-TO-BACK WRITE/READ CYCLES
It is important to note that there are specific restrictions on the timing of back-to-back write-read operations. These
restrictions concern reading the control registers after any write cycle to the LAN9117 device. In many cases there is a
required minimum delay between writing to the LAN9117, and the subsequent side effect (change in the control register
value). For example, when writing to the TX Data FIFO, it takes up to 135ns for the level indication to change in the
TX_FIFO_INF register.
In order to prevent the host from reading stale data after a write operation, minimum wait periods must be enforced.
These periods are specified in Table 6-1, "Read After Write Timing Rules". The host processor is required to wait the
specified period of time after any write to the LAN9117 before reading the resource specified in the table. These wait
periods are for read operations that immediately follow any write cycle. Note that the required wait period is dependent
upon the register being read after the write.
Performing "dummy" reads of the BYTE_TEST register is a convenient way to ensure that the minimum write-to-read
timing restriction is met. Table 6-1 also shows the number of dummy reads that are required before reading the register
indicated. The number of BYTE_TEST reads in this table is based on the minimum timing for Tcycle (45ns). For micro-
processors with slower busses the number of reads may be reduced as long as the total time is equal to, or greater than
the time specified in the table. Note that dummy reads of the BYTE_TEST register are not required as long as the min-
imum time period is met.
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