EM260-RTR datasheet(23/50 Pages) SILICONIMAGE

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EM260

Page 23

120-0260-000M Rev 1.1

5.2.1

Command Section

The Host begins the transaction by asserting the Slave Select and then sending a command to the EM260. This

command can be of any length from 2 to 136 bytes and must not begin with 0xFF. During the Command

section, the EM260 will respond with only 0xFF. The Host should ignore data on MISO during the Command

section. Once the Host has completed transmission of the entire message, the transaction moves to the Wait

section.

5.2.2

Wait Section

The Wait section is a period of time during which the EM260 may be processing the command or performing

other operations. Note that this section can be any length of time up to 300 milliseconds. Because of the

variable size of the Wait section, an interrupt-driven or polling-driven method is suggested for clocking the SPI

as opposed to a DMA method. Since the EM260 can require up to 300 milliseconds to respond, as long as the

Host keeps Slave Select active, the Host can perform other tasks while waiting for a Response.

To determine when a Response is ready, use one of two methods:

Clock the SPI until the EM260 transmits a byte other than 0xFF.

Interrupt on the falling edge of nHOST_INT.

The first method, clocking the SPI, is recommended due to simplicity in implementing. During the Wait

section, the EM260 will transmit only 0xFF and will ignore all incoming data until the Response is ready. When

the EM260 transmits a byte other than 0xFF, the transaction has officially moved into the Response section.

Therefore, the Host can poll for a Response by continuing to clock the SPI by transmitting 0xFF and waiting for

the EM260 to transmit a byte other than 0xFF. The EM260 will also indicate that a Response is ready by

asserting the nHOST_INT signal. The falling edge of nHOST_INT is the indication that a Response is ready. Once

the nHOST_INT signal asserts, nHOST_INT will return to idle after the Host begins to clock data.

5.2.3

Response Section

When the EM260 transmits a byte other than 0xFF, the transaction has officially moved into the Response

section. The data format is the same format used in the Command section. The response can be of any length

from 2 to 136 bytes and will not begin with 0xFF. Depending on the actual response, the length of the

response is known from the first or second byte and this length should be used by the Host to clock out exactly

the correct number of bytes. Once all bytes have been clocked, it is allowable for the Host to deassert chip

select. Since the Host is in control of clocking the SPI, there are no ACKs or similar signals needed back from

the Host because the EM260 will assume the Host could accept the bytes being clocked on the SPI. After every

transaction, the Host must hold the Slave Select high for a minimum of 1ms. This timing requirement is called

the inter-command spacing and is necessary to allow the EM260 to process a command and become ready to

accept a new command.

5.2.4

Asynchronous Signaling

When the EM260 has data to send to the Host, it will assert the nHOST_INT signal. The nHOST_INT signal is

designed to be an edge-triggered signal as opposed to a level-triggered signal; therefore, the falling edge of

nHOST_INT is the true indicator of data availability. The Host then has the responsibility to initiate a

transaction to ask the EM260 for its output. The Host should initiate this transaction as soon as possible to

prevent possible backup of data in the EM260. The EM260 will deassert the nHOST_INT signal after receiving a

byte on the SPI. Due to inherent latency in the EM260, the timing of when the nHOST_INT signal returns to idle

can vary between transactions. nHOST_INT will always return to idle for a minimum of 10us before asserting

again. If the EM260 has more output available after the transaction has completed, the nHOST_INT signal will

assert again after Slave Select is deasserted and the Host must make another request.

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