Jameco Electronics 2000 Timer and Clock Usage, Write-only Registers Without Shadow Registers

ld hl,PDDDRShadow		; point to shadow register
ld de,PDDDR		; set de to	point to I/O reg
set	5,(hl)	; set bit 5	of shadow	register
; use ldd instruction for atomic			transfer
ioi	ldd	; (io de)<-(hl) side		effect: hl--, de--

In this case, the ldd instruction when used with an I/O prefix provides a convenient data move from a memory location to an I/O location. Importantly, the ldd instruction is an atomic operation so there is no danger that an interrupt routine could change the shadow register during the move to the PDDDR register.

18.3.2.2 Non-atomic Instructions

If the following two instructions were used instead of the ldd instruction,

ld a,(hl)

ld (PDDDR),a ; output to PDDDR

then an interrupt could take place after the first instruction, change the shadow register and the PDDDR register, and then after a return from the interrupt, the second instruction would execute and store an obsolete copy of the shadow register in the PDDDR, setting it to a wrong value.

18.3.3 Write-only Registers Without Shadow Registers

Shadow register are not needed for many of the registers that can be written to. In some cases, writing to registers is used as a handy way of changing a peripheral’s state, and the data bits written are ignored. For example, a write to the status register in the Rabbit serial ports is used to clear the transmitter interrupt request, but the data bits are ignored, and the status register is actually a read-only register except for the special functionality attached to the act of writing the register. An illustration of a write-only register for which a shadow is unnecessary is the transmitter data register in the Rabbit serial port. The transmitter data register is a write-only register, but there is little reason to have a shadow register since any data bits stored are transmitted promptly on the serial port.

18.4 Timer and Clock Usage

The battery-backable real-time clock is a 48 bit counter that counts at 32768 counts per second. The counting frequency comes from the 32.768 kHz oscillator which is separate from the main oscillator. Two other important devices are also powered from the 32.768 kHz oscillator: the periodic interrupt and the watchdog timer. It is assumed that all mea- surements of time will derive from the real-time clock and not the main processor clock which operates at a much higher frequency (e.g. 22.1184 MHz). This allows the main pro- cessor oscillator to use less expensive ceramic resonators rather than quartz crystals. Ceramic resonators typically have an error of 5 parts in 1000, while crystals are much more accurate, to a few seconds per day.

244 Rabbit 3000 Microprocessor