eZ80L92 MCU
Product Specification
167
When using the ZDI interface, any Write or Read operations of peripheral registers in the
I/O address space produces the same effect as Read or Write operations using the CPU.
Because many register Read/Write operations exhibit secondary effects, such as clearing
flags or causing operations to commence, the effects of the Read/Write operations during a
ZDI Break must be taken into consideration.
Bus Requests During ZDI Debug Mode
The ZDI block on the ZLP12840 allows an external device to take control of the address
and data bus while the ZLP12840 is in DEBUG mode. ZDI_BUSACK_EN causes ZDI to
allow or prevent acknowledgement of bus requests by external peripherals. The bus
acknowledge only occurs at the end of the current ZDI operation (indicated by a High dur-
ing the single-bit byte separator). The default reset condition is for bus acknowledgement
to be disabled. To allow bus acknowledgement, the ZDI_BUSACK_EN must be written.
When an external bus request (BUSREQ pin asserted) is detected, ZDI waits until comple-
tion of the current operation before responding. ZDI acknowledges the bus request by
asserting the bus acknowledge (BUSACK) signal. If the ZDI block is not currently shift-
ing data, it acknowledges the bus request immediately. ZDI uses the single-bit byte separa-
tor of each data word to determine if it is at the end of a ZDI operation. If the bit is a
logical 0, ZDI does not assert BUSACK to allow additional data Read or Write operations.
If the bit is a logical 1, indicating completion of the ZDI commands, BUSACK is asserted.
Potential Hazards of Enabling Bus Requests During Debug Mode
There are some potential hazards that the user must be aware of when enabling external
bus requests during ZDI Debug mode. First, when the address and data bus are being used
by an external source, ZDI must only access ZDI registers and internal CPU registers to
prevent possible Bus contention. The bus acknowledge status is reported in the
ZDI_BUS_STAT register. The BUSACK output pin also indicates the bus acknowledge
state.
A second hazard is that when a bus acknowledge is granted, the ZDI is subject to any
WAIT states that are assigned to the device currently being accessed by the external
peripheral. To prevent data errors, ZDI should avoid data transmission while another
device is controlling the bus.
Finally, exiting ZDI Debug mode while an external peripheral controls the address and
data buses, as indicated by BUSACK assertion, may produce unpredictable results.
PS013015-0316
Zilog Debug Interface
ZILOG [ ZILOG, INC. ]
