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04/2010

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Figure 7: SPI BUS Timing Diagram

The MSB data out on MISO is valid soon after CSB goes low. The MOSI data is valid soon after the falling edge of

SCK. The Encoder drives data out on MISO as long as CSB is low.

Normally, CSB goes low, then after 8 clocks the command is interpreted. CSB high resets the

clock counter, and terminates any command sequence.

SPI Commands:

The commands are all 8 bits long, the MSB is shifted in first, and is the leftmost bit shown in the documentation.

Encoder Protocol Considerations:

The Encoder is designed to operate with a high speed SPI link, in full duplex mode. This implies the host can is-

sue commands and read data as quickly as necessary but there has to be an acknowledgement from the slave just

before the data is transferred.

Essentially the host issues a command, receives zero or more wait sequences (0xA5 or 1010,0101) then the echo

of the command followed by an optional payload.

So, for example to read the position, the host issues rd_pos (0x10 or 0001,0000), receiving a series of wait

sequences (0xA5) then a reflected rd_pos (0x10), then the MSB data followed by the LSB data.

It is recommended that the host leave a 20 us gap between reads to avoid extending the read time by forcing wait

sequences.

Command 0x00: nop_a5

This command is ignored by the Encoder and simply causes the next data to be read. The encoder responds with

0xA5 if there is nothing else to send.

Command 0x10: rd_pos

This command causes a read of the current position.

The sequence is as follows:

1) issue read command, receive idle character

2) issue NOP, receive idle character 0xA5 or 0x10

3) repeat step 2 if it is 0xA5

4) issue NOP and receive MSB position

5) issue NOP and receive LSB position

Note that it is possible to overlap commands, so instead of NOP is several steps above

the user could start another operation. The read and write FIFOs for the PCI streams are

16 bytes long and it is up to the user to avoid overflow.

phone 503.612.2300

fax 503.612.2380 www.cui.com

PART NUMBER:

DESCRIPTION:

AMT203

12 BIT ABSOLUTE ENCODER