CUI DEVICES | SERIES: AMT20 | DESCRIPTION: MODULAR ABSOLUTE ENCODER

date 08/05/2022 | page 9 of 10

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APPLICATION NOTES (CONTINUED)

All commands are 8 bits long. The msb is shifted in first, and is the leftmost bit shown in Figure 1.

Encoder Protocol Considerations:

The encoder is designed to operate with a high speed SPI link, in full duplex mode. This implies the host can issue commands and read data as quickly

as necessary but there has to be an acknowledgement from the slave just before the data is transferred.

First the host will issue a command, then the encoder may respond with wait responses (0xA5) until ready. Once ready the encoder will echo the original

command received from the master, followed by the requested data.

For example, to read the position the master will send the rd_pos command (0x10), it will then send no operation commands (nop_a5, 0x00) until it

receives the original rd_pos command back. Once it receives the rd_pos response from the encoder, it knows that the next two bytes of data will be the

MSB and the LSB respectively. To receive those two bytes the master will send two no operation commands.

It is recommended that the master leave a 20

μs delay between reads to avoid extending the read time by forcing wait sequences.

Each byte transmitted must be followed by a release of the Chip Select line (CSB).

Command 0x00: nop_a5 (no operation)

This no operation command is ignored by the encoder and simply causes the next byte of data to be read. The encoder will respond with 0xA5 if there is

no remaining data to be sent.

Command 0x10: rd_pos (read position)

This command causes a read of the current position.

To read position this sequence should be followed:

1.

Master sends rd_pos command. Encoder responds with idle character.

2.

Continue sending nop_a5 command while encoder response is 0xA5

3.

If response was 0x10 (rd_pos), send nop_a5 and receive MSB position (lower 4 bits of this byte are the upper 4 of the 12-bit

position)

4.

Send second nop_a5 command and receive LSB position (lower 8 bits of 12-bit positon)

Note that it is possible to overlap commands. For instance, instead of issuing nop_a5 for steps 3 and 4, you could begin another read position sequence

since the position data is already in the buffer. The read and write FIFOs for the streams are 16 bytes long and it is up to the user to avoid overflow.

Command 0x70: set_zero_point (zero set)

This command sets the current position to zero and saves this setting in the EEPROM.

To set the zero point this sequence should be followed:

1.

Send set_zero_point command

2.

Send nop_a5 command while response is not 0x80

3.

A response of 0x80 means that the zero set was successful and the new position offset is stored in EEPROM.

4.

The encoder must be power cycled. If the encoder is not power cycled, the position values will not be calculated off the latest zero position.

When the encoder is powered on next the new offset will be used for the position calculation.

SPI COMMANDS

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