SPI¶
The Duckbill SPI is intended to be used in the evaluation kit “PLC bundle for Internet of Things”. Thus the device tree file contains a section which refers to the Qualcomm Atheros QCA7000 serial-to-powerline bridge.
The settings for the QCA driver are listed in the following table.
Attribute | Source | SSP2 |
---|---|---|
Type | Modprobe config | External |
MAC address | Device tree | Value from OTP |
Max SPI clock rate | Device tree | 8 MHz |
Actual SPI clock rate | Clock tree | 6 MHz |
SPI mode | Device tree | 3 |
Expected SPI slave | Device tree | QCA7000 |
Expected command mode | Device tree | burst |
The device tree binding for the QCA driver can be found in the Kernel sources: Documentation/devicetree/bindings/net/qca-qca7000-spi.txt
Note 1: The MAC address is added dynamically by U-Boot into the device tree.
Note 2: The reference clock for SSP2 is switched from ref_io1 to ref_xtal by an additional kernel patch. This limits the maximum SPI frequency to 12 MHz. This is due to observed bit errors when ref_io1 is used as clock source. However, for the use-case with QCA7000, the maximum clock rate is already limited by the QCA7000 which only supports up to 12 MHz. The reason for the actual 6 MHz is, that the SPI leaves the Duckbill via the connector and “flying wires”.
Due the limitation of the selected clock and SPI the following SPI clock rates are available:
- 12.0 MHz
- 6.0 MHz
- 4.0 MHz
- 3.0 MHz
- 2.4 MHz
- 2.0 MHz
- 1.72 MHz
- 1.5 MHz
- 1.34 MHz
- 1.2 MHz
- 1.1 MHz
- 1.0 MHz