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Unformatted text preview: ication, corresponding to the communication receive and transmit functions, one for each counter-timer and one which can be generated directly by software (principally to enable an FIQ handler to generate an IRQ). Two 16-bit counter-timers are required, though more may be added. These are controlled by registers with fixed offsets relative to CTBase. The counters operate from the system clock with selectable pre-scaling of 0, 4 or 8 bits (so the input frequency is the system clock frequency divided by 1, 16 or 256). Each counter-timer has a control register which selects the pre-scaling, enables or disables the counter and specifies the mode of operation as free-running or periodic, and a load register which specifies the value that the count starts from. A write to the 'load' register initializes the count value, which is then decremented to zero when an interrupt is generated. A write to the 'clear' register clears the interrupt. In free-running mode the counter continues to decrement past zero, whereas in periodic mode it is reloaded with the value in the 'load' register and decrements from there. The current count value may be read from the 'value' register at any time. The reset and pause controller includes registers which are addressed at fixed offsets from RPCBase. The readable registers give identification and reset status information, including whether or not a power-on reset has occurred. The writeable registers can set or clear the reset status (though not the power-on reset status bit; this can only be set by a hardware power-on reset), clear the reset map (for instance to switch the ROM from location zero, where it is needed after power-on for the ARM reset vector, to the normal memory map), and put the system into pause mode where it uses minimal power until an interrupt wakes it up again. Any ARM system which incorporates this basic set of components can support a suitably configured operating system kernel. System design then consists of adding further application-specific peripherals and software, building upwards from a functional base. Since most applications require t...
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This document was uploaded on 10/30/2011 for the course CSE 378 380 at SUNY Buffalo.
- Spring '09