saa5355 NXP Semiconductors, saa5355 Datasheet - Page 24

• Manufacturer Part Number: saa5355
• Description: Single-chip Colour Crt Controller Ftfrom
• Manufacturer: NXP Semiconductors

Philips Semiconductors
Microprocessor access to register map
FTFROM has a set of internal registers which, when memory-mapped, behave as an 8-bit wide RAM connected to the
upper part of the data bus (Fig.20). The control signals UDS and R/W are reversed to become inputs and the register
map is enabled by the signal RE. Addresses are input via the lower part of the bus. A data transfer acknowledge signal
(DTACK) indicates to the microprocessor that the data transfer is complete.
The main data and address paths used in a connected 68000 interface are shown in Fig.21. The outputs from the octal
latches (74LS373) are enabled only when the 68000 has made the bus available in response to a bus request (BR).
When the register map is accessed data is transferred via the upper part of the bus and the microprocessor’s low-order
address is passed to FTFROM via the octal buffers (74LS244). At the same time the bidirectional buffers (74LS245)
disable the signals from the low order data bus of the 68000.
The buffers ‘244 and ‘245 may be omitted in a 16-bit write-only configuration where the least-significant data byte is
interpreted by FTFROM as an address. Here it will generally be necessary for the microprocessor to hold a (readable)
‘master copy’ of FTFROM’s scroll map contents at a location in its main memory.
8-bit microprocessors
Although the control bus is optimised for the SCN68000 16-bit microprocessor unit, FTFROM will operate with a number
of widely differing industry-standard 8, 16 or more-bit microprocessors or microcontrollers (e.g. SCN68008, MAB8051).
The interfacing of 8-bit microprocessors to the 16-bit wide display memory is made simple by FTFROM’s on-chip
link-through buffer which provides the microprocessor with bidirectional access to the lower (odd) half of the memory.
The link-through buffer is enabled by the buffer-enable signal BUFEN, and the send/receive direction is controlled by the
signal S/R.
The main data and address paths used in a connected 8-bit microprocessor system are shown in Fig.22. The interface
is similar to that of the 16-bit system but here the display memory does not receive A0 as an address, rather A0 is used
as the major enabling signal for BUFEN (enables when HIGH).
March 1986
Single-chip colour CRT controller
(FTFROM)
Fig.20 Microprocessor access to register map.
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Product specification
SAA5355