Z8F021ASH020SC datasheet(184/276 Pages) ZILOG

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PS022815-0206

Non-Volatile Data Storage

Z8 Encore! XP® 4K Series

Product Specification

165

Byte Read

To read a byte from the NVDS array, user code must first push the address onto the stack.

User code issues a CALL instruction to the address of the byte-read routine (0x1000). At

the return from the sub-routine, the read byte resides in working register R0, and the read

status byte resides in working register R1. The contents of the status byte are undefined for

read operations to illegal addresses. Also, the user code should pop the address byte off the

stack.

The read routine uses 9 bytes of stack space in addition to the one byte of address pushed

by the user. Sufficient memory must be available for this stack usage.

Because of the Flash memory architecture, NVDS reads exhibit a non-uniform execution

time. A read operation takes between 44

μs and 489 μs (assuming a 20 MHz system

clock). Slower system clock speeds result in proportionally higher execution times.

NVDS byte reads from invalid addresses (those exceeding the NVDS array size) return

0xff. Illegal read operations have a 2

μs execution time.

The status byte returned by the NVDS read routine is zero for successful read, as deter-

mined by a CRC check. If the status byte is non-zero, there was a corrupted value in the

NVDS array at the location being read. In this case, the value returned in R0 is the byte

most recently written to the array that does not have a CRC error.

Power Failure Protection

The NVDS routines employ error checking mechanisms to ensure a power failure endan-

gers only the most recently written byte. Bytes previously written to the array are not per-

turbed.

A system reset (such as a pin reset or watchdog timer reset) that occurs during a write

operation also perturbs the byte currently being written. All other bytes in the array are

unperturbed.

Optimizing NVDS Memory Usage for Execution Speed

As Table 107 shows, the NVDS read time varies drastically, this discrepancy being a

trade-off for minimizing the frequency of writes that require post-write page erases. The

NVDS read time of address N is a function of the number of writes to addresses other than

N since the most recent write to address N, as well as the number of writes since the most

recent page erase. Neglecting effects caused by page erases and results caused by the ini-

tial condition in which the NVDS is blank, a rule of thumb is that every write since the

most recent page erase causes read times of unwritten addresses to increase by 1

μs, up to

a maximum of (511-NVDS_SIZE)

μs.

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