FPGA Reusables
Reusable Components
Several components were found useful as reusable units rather than unique modules. They are listed here roughly from the low level units to high level "wrappers".
Read Request (reqFromAddr)
Sends request for reading (R/W pin high) to the Transceiver at the specified address. (Optionally "Go" pulse can be appropriately delayed in this module to ensure that parameter signals sent to transceiver have gone through their transitions. Currently, these provisions are commented out, relying on clock falling edge latching of the sent data.) This unit is not intended for direct use, but rather to be the core of the fetcher components described here.
Write Request (wrToAddr)
Sends a write request (R/W pin low) to transceiver at the specified address with the specified byte of data. The primary reason of this instruction "packaging" was to obscure the complexity of appropriate delays to ensure proper signal latching. The delayed version is currently commented out under the assumption of falling edge of clock latching of data by the Transceiver.
2-byte Write Request (wr2BtoAddr)
Coordinates write operations to two adjacent 1-byte registers in the Ethernet Controller chip. This is an essential abstraction because the controller often requires 16-bit exchanges especially when specifying 16-bit buffer locations in random memory access. Only the address of the first register needs to be passed in.
Byte Fetcher (getByte)
Fetches a byte of data from the Ethernet Controller registers via the Transceiver. This unit creates yet another abstraction layer "in series" with the transceiver. The two step process of sending a request with all accompanying parameters and waiting for reply is now a matter of a single "call". The actual request is performed via the reqFromAddr unit described above, so the timing characteristics are inherited from that module.
Auto-reader (AutoRd)
Performs the same function as the Byte Fetcher (described above) but is hard-wired to address the RXAUTORD (0x01). This is convenient because the Ethernet Controller's AutoRead interface will be used extensively to parse through packets. AutoRd is in fact just a wrapper around GetByte passing through all signals accept the fixed address input.
Random Access Write (RAwrToAddr)
Writing directly to Ethernet Controller RAM (not to be confused with the control registers) in random access mode requires more work. A 16-bit address needs to be passed in 8 bits at a time to appropriate control registers and the actual 8-bit data is sent on the third transaction. Because the buffer is sometimes loaded in sequentially, this unit is designed to just increment the address of the previous random access write if the passed in address has a '1' in the MSB. (This would never be a legal address, as it is out of range.) The calling module, therefore, does not need to store the current write location of the RAM. wr2BtoAddr and wrToAddr are very convenient for these three transactions.
2-byte Random Access Write (RAwr2BtoAddr)
Because the data values exchanged between the board and the PC range from 10 to 14 bit depth, zero padded 16-bit words will be used. So, to load this 2-byte word into the transfer buffer requires two random access writes. This module packages this functionality by including two calls to RAwrToAddr with address increments and input data latching taken into account.