SN260 datasheet(23/88 Pages) STMICROELECTRONICS

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SN260

SPI protocol

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6.2.1

Command section

The Host begins the transaction by asserting the Slave Select and then sending a command

to the SN260. This command can be of any length from 2 to 128 bytes and must not begin

with 0xFF. During the Command section, the SN260 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.

6.2.2

Wait section

The Wait section is a period of time during which the SN260 may be processing the

command or performing other operations. Note that this section can be any length of time up

to 200 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 SN260 can require up to 200 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 SN260 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 SN260 will transmit only 0xFF and will ignore all incoming data

until the Response is ready. When the SN260 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 SN260 to

transmit a byte other than 0xFF. The SN260 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.

6.2.3

Response section

When the SN260 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 128 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 de-assert

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 SN260 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 SN260 to process a command and become ready to

accept a new command.

6.2.4

Asynchronous signaling

When the SN260 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 SN260 for

its output. The Host should initiate this transaction as soon as possible to prevent possible

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