Calibre UK UCA93 - Manual (Page 23)

CALIBRE
Issue 1.0 Page 20
01/07/03
The slave functions of the UCA93 differ most from traditional Calibre AT and PCI bus based products.
A user must make a pre-determined decision whether to put the adapter into master, slave transmitter
or slave receiver modes. The adapter’s own address is established as part of the Setup function. If the
adapter is in master mode (the default) , it will not respond to its own address if this is generated by
another master on the system. If the adapter is in either slave mode it will recognise its own address
and an acknowledge will be generated on the I2C bus.
Slave Transmitter Function
When the BlockSlaveTransmittter function is called the user sends a data block of selectable size (max
2048bytes) into a buffer together a timeout time in seconds. When a master generates the adapter’s
own address (either write or read) the adapter will generate an acknowledge on the I2C bus.
If the address was even (master write) the adapter will assume that the master wishes to send a pointer
to a location in the data block. The pointer can be one or two bytes. If the pointer is single the upper
byte will default to 0x00. If the master fails to send a pointer both bytes will default to 0x00 and if the
master send more than two bytes on the last two will be used – the others will be discarded. Data in the
block will then be discarded so that the first byte aligns with the pointer sent. Note that the adapter will
hold SCL low during this alignment which slows down the I2C bus – the master should be able to
tolerate this.
If the address was odd (master read) the adapter will assume that the master does not wish to send a
pointer and that data transmission should start from the beginning of the data block.
If the master has sent a pointer the master should then generate a restart followed by the adapter’s read
address which will be acknowledged.
Data is then transmitted from the adapter to the master starting at the pointer location. Remembering
that the master is in charge of the length of the transfer and the generation of the stop, the adapter will
transmit data to the master for as long as required. If the transfer is longer than the available data the
last available byte in the block will be repeated; if it is shorter then not all the data in the block will be
used.
When the transmission is terminated by the master the adapter will send a single byte back to the host.
The BlockSlaveTransmitterStatus function waits for this byte and the byte is interpreted as described
the function definition. Note that this byte may return either as a result of a timeout (when the master
fails to initiate a transfer to the adapter within the timeout seconds) or the master correctly terminating
the transfer.
When the BlockSlaveTransmittter function terminates the adapter goes back into an idle state – it will
not acknowledge its own address or repeat any other slave behaviour until the slave function is called
again.
Users will note that the BlockSlaveTransmittter function is designed as much as possible to emulate the
behaviour of an EEPROM. The only differences are that the block size is limited to 2048 byte, there
may be some delay on the bus white data is aligned with the pointer and the function only runs once
without being recalled. The function is also designed to keep the transmission going even if a mistake
is made in the master’s protocol although it is important that the master generates a stop eventually.
Slave Receiver Function
The BlockSlaveReceiver function is simpler than the previous function! When it is called the user must
instruct the adapter on how many bytes it wishes to receive back (max 2048) and on a timeout in
seconds.
When the master generates the adapter’s own address the adapter will generate an acknowledge on
the I2C bus. Since the adapter is about to receive data this should be an even (write) address.
The master then transmits data to the adapter and the adapter will acknowledge each byte then put it
into the pre-determined size buffer to be transmitted to the host. The adapter will accept data from the
master for as long as the master sustains the transfer. If too much data is sent by the master, the
adapter keeps on acknowledging the master but stops putting data into the buffer. Likewise, if the
master terminates the transfer before the buffer is full, the adapter pads out the buffer with 0xFFs to the
predetermined size.