Error Conditions - ZiLOG Z8 User Manual

Z8 Microcontrollers
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2.2 Z8 MCU STANDARD REGISTER FILE (Continued)
Note: The full register file is shown. Please refer to the selected device product specification for actual file size.

2.2.4 Error Conditions

Registers in the Z8 Standard Register File must be correctly used
because certain conditions produce inconsistent results and
should be avoided.
• Registers F3H and F5H-F9H are write-only registers. If an
attempt is made to read these registers, FFH is returned.
Reading any write-only register will return FFH.
• When register FDH (Register Pointer) is read, the least
significant four bits (lower nibble) will indicate the current
Expanded Register File Bank. (Example: 0000 indicates the
Standard Register File, while 1010 indicates Expanded
Register File Bank A.)
When Ports 0 and 1 are defined as address outputs, registers
•
00H and 01H will return 1s in each address bit location when
read.
2-4
R7 R6 R5 R4 R3 R2 R1 R0
The upper nibble of the register file address,
provided by the register pointer, specifies
the active working-register group.
FF
Working Register Group F
F0
EF
80
7F
70
6F
60
5F
50
4F
40
3F
Specified Working Register Group
30
2F
20
1F
Working Register Group 1
10
0F
Working Register Group 0
I/O Ports
00
Figure 2-4. Register Pointer
R253
(Register Pointer)
The lower nibble
of the register
file address
(provided by the
instruction) points
to the specified
register.
R15 to R0
R15 to R4
R3 to R0
Writing to bits that are defined as timer output, serial output,
•
or handshake output will have no effect.
The Z8 instruction DJNZ uses any general-purpose working
•
register as a counter.
Logical instructions such as OR and AND require that the
•
current contents of the operand be read. They therefore will
not function properly on write-only registers.
The WDTMR register must be written within the first 60
•
internal system clocks (SCLK) of operation after a reset.
ZiLOG
UM001601-0803