AMT203 Datasheet, PDF (6/8 Pages) CUI INC – 12 BIT ABSOLUTE ENCODER

PART NUMBER: AMT203
date 04/2010
page 6 of 8
DESCRIPTION: 12 BIT ABSOLUTE ENCODER
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.
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