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ZEUS Technical Manual Power and power management
© 2007 Eurotech Ltd Issue D 72
CAN bus power management
The Microchip MCP2515 CAN bus controller has an internal sleep mode that is used to
minimize the current consumption of the device. The SPI interface remains active for
reading even when the MCP2515 is in sleep mode, allowing access to all registers. To
enter sleep mode, the mode request bits are set in the CANCTRL register
(REQOP<2:0>).
When in internal sleep mode, the wake-up interrupt is still active (if enabled) which
means that upon detecting activity on the bus, it can be used as a wake-up source for
PXA270. When in sleep mode, the MCP2515 stops its internal oscillator. The
MCP2515 will wake up when bus activity occurs, or when the PXA270 sets the
CANINTF.WAKIF bit to ‘generate’ a wake-up attempt, via the SPI interface. The power
consumption in sleep mode is near zero (15μW).
The transceiver device (MCP2551) may be placed in sleep mode by applying a high-
level to GPIO2 of Expander 2 (I2C address 0x22, signal CAN_SHDN). In sleep mode,
the transmitter is switched off and the receiver operates at a lower current. The receive
pin on the controller side (RXD) is still functional but will operate at a slower rate. The
power consumption in sleep mode is 1.8mW.
EXP2_GPIO2 (CAN_SHDN) MCP2551 operation status
0 Normal operation
1 Sleep mode
For more information about power management, refer to the MCP2515 and MCP2551
datasheets contained on the Development Kit CD.
Clock generator power management
Two clock synthesizer IC’s (Cypress CY22381) can be placed in low power mode by
shutting down the clock outputs in case the corresponding interfaces are not used. You
can do this by applying a low-level to the GPIO0 pin on the Expander 2 (I2C address
0x22, signal CLK_SHDOWN#). The following clocks are affected:
• ST16C554Q UART 14.7456MHz clock.
• Ethernet PHY 25MHz clock.
• PC/104 8MHz and 14MHz clocks.
• CAN bus 16MHz clock.
• Audio clock 24.576MHz.
This can reduce the power consumption of each clock generator IC down to 66 μW.
EXP2_GPIO0 (CLK_SHDOWN#) Clock operation status
0 Shutdown mode
1 Normal operation