ZBasic System Library
52
ZBasic Microcontrollers
Type
Subroutine
Invocation
BusRead(addr, data, count)
BusRead(addr, data, count, delta)
Parameter
Method
Type
Description
addr
ByVal
integral
The bus address at which to begin reading.
data
ByRef
anyType
A buffer to receive the data read.
count
ByVal
integral
The number of bytes to read.
delta
ByVal
integral
The amount by which the address should be changed after
each byte is read.
Discussion
For ZBasic devices that support external RAM (e.g. ZX-1281), if the external RAM interface is enabled
and a bus has not been defined using DefineBus(), then the external RAM interface is used for the read
operation. In this case, the full 16 bits of the specified address are used and the delta parameter is
interpreted as a signed 8-bit value that is sign-extended before adding it to the address with each
iteration.
For ZBasic devices that do not support external RAM or if the external RAM interface is not enabled, this
routine performs a series of read operations on the bus previously defined with the DefineBus() call. This
is called the “bit bang” mode. For each read cycle, the low 8-bits of the address is output on the
previously specified port and then the ALE pin is strobed (high, then back low). Next, the port is made an
input and the RD pin is set low, data is read via the PIN register corresponding to the port, and the RD pin
is set back high again. The data value read is stored in the buffer, the specified delta is added to the 8-bit
bus address and the cycle is repeated until the specified number of bytes has been read.
It is important to remember that in the bit bang mode only 8 bits of the address are used. Depending on
the values of the addr, count and delta parameters, the effective address may wrap around to zero.
For example, with delta=1 specifying a count parameter larger than (256 – LoByte(addr)) will
result in the effective address wrapping around to zero.
In either mode, if the optional delta parameter is not specified, the value of 1 is assumed. Specifying
the delta as zero will result in multiple reads from the same address. A delta of –¹ or &Hff will result in
the address being decremented after each read.
Example
Dim data(1 to 20) as Byte
Call DefineBus(Port.A, C.0, C.1, C.2)
Call BusRead(0, data, SizeOf(data))
Compatibility
This subroutine is not available on ZX models that are based on the ATmega32 processor (e.g. the ZX-
24) nor is is available on ATxmega-based ZBasic devices. Moreover, it is not available in BasicX
compatibility mode.
See Also
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