COM20019I-HT datasheet(35/65 Pages) SMSC

35 / 65 page

Low Cost ARCNET (ANSI 878.1) Controller with 2K x 8 On-Board RAM

SMSC COM20019I

Page 35

Rev. 03-07-06

DATASHEET

channeled to and from the microcontroller via the 8-bit data register. For example: a packet in the internal

RAM buffer is read by the microcontroller by writing the corresponding address into the Address Pointer

High and Low Registers (offsets 02H and 03H).

Note that the High Register should be written first,

followed by the Low Register, because writing to the Low Register loads the address. At this point the

device accesses that location and places the corresponding data into the data register.

The

microcontroller then reads the data register (offset 04H) to obtain the data at the specified location. If the

Auto Increment bit is set to logic "1", the device will automatically increment the address and place the

next byte of data into the data register, again to be read by the microcontroller. This process is continued

until the entire packet is read out of RAM. Refer to Figure 7 for an illustration of the Sequential Access

operation. When switching between reads and writes, the pointer must first be written with the starting

address.

At least one cycle time should separate the pointer being loaded and the first read (see timing

parameters).

6.3.2

Access Speed

The COM20019I is able to accommodate very fast access cycles to its registers and buffers. Arbitration to

the buffer does not slow down the cycle because the pointer based access method allows data to be

prefetched from memory and stored in a temporary register. Likewise, data to be written is stored in the

temporary register and then written to memory.

For systems which do not require quick access time, the arbitration clock may be slowed down by setting

bit 0 of the Setup1 Register equal to logic "1". Since the Slow Arbitration feature divides the input clock by

two, the duty cycle of the input clock may be relaxed.

6.4

SOFTWARE INTERFACE

The microcontroller interfaces to the COM20019I via software by accessing the various registers. These

actions are described in the Internal Registers section. The software flow for accessing the data buffer is

based on the Sequential Access scheme. The basic sequence is as follows:

Disable Interrupts

Write to Pointer Register High (specifying Auto-Increment mode)

Write to Pointer Register Low (this loads the address)

Enable Interrupts

Read or Write the Data Register (repeat as many times as necessary to empty or fill the buffer)

The pointer may now be read to determine how many transfers were completed.

The software flow for controlling the Configuration, Node ID, Tentative ID, and Next ID registers is

generally limited to the initialization sequence and the maintenance of the network map.

Additionally, it is necessary to understand the details of how the other Internal Registers are used in the

transmit and receive sequences and to know how the internal RAM buffer is properly set up.

The

sequence of events that tie these actions together is discussed as follows.

6.4.1

Selecting RAM Page Size

During normal operation, the 2K x 8 of RAM is divided into four pages of 512 bytes each. The page to be

used is specified in the "Enable Transmit (Receive) from (to) Page fnn" command, where "nn" specifies

page 0, 1, 2, or 3. This allows the user to have constant control over the allocation of RAM.

When the Offset bit "f" (bit 5 of the "Enable Transmit (Receive) from (to) Page fnn" command word) is set

to logic "1", an offset of 256 bytes is added to the page specified. For example: to transmit from the second

half of page 0, the command "Enable Transmit from Page fnn" (fnn=100 in this case) is issued by writing

0010 0011 to the Command Register. This allows a finer resolution of the buffer pages without affecting

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