Rainbow Electronics AT75C310 - Manual (Page 102)

AT75C310
102
Master Mode
In master mode, the SPI controls data transfers to and from
the slave(s) connected to the SPI bus. The SPI drives the
chip select(s) to the slave(s) and the serial clock (SPCK).
After enabling the SPI, a data transfer begins when the
ARM core writes to the SP_TDR (Transmit Data Register).
See Table 21.
Transmit and receive buffers maintain the data flow at a
constant rate with a reduced requirement for high-priority
interrupt servicing. When new data is available in the
SP_TDR (Transmit Data Register) the SPI continues to
transfer data. If the SP_RDR (Receive Data Register) has
not been read before new data is received, the Overrun
Error (OVRES) flag is set.
The delay between the activation of the chip select and the
start of the data transfer (DLYBS), as well as the delay
between each data transfer (DLYBCT), can be pro-
grammed for each of the four external chip selects. All data
transfer characteristics including the two timing values are
programmed in registers SP_CSR0 to SP_CSR(Chip
Select Registers). See Table 21 on page 107.
In master mode, the peripheral selection can be defined in
two different ways:
1. Fixed Peripheral Select: SPI exchanges data with
only one peripheral
2. Variable Peripheral Select: Data can be exchanged
with more than one peripheral
Figures 22 and 23 show the operation of the SPI in master
mode. For details concerning the flag and control bits in
these diagrams, see Table 21 on page 107.
Fixed Peripheral Select
This mode is ideal for transferring memory blocks without
the extra overhead in the transmit data register to deter-
mine the peripheral.
Fixed peripheral select is activated by setting bit PS to zero
in SP_MR (Mode Register). The peripheral is defined by
the PCS field that is also in SP_MR.
This option is only available when the SPI is programmed
in master mode.
Variable Peripheral Select
Variable peripheral select is activated by setting bit PS to
one. The PCS field in SP_TDR (Transmit Data Register) is
used to select the destination peripheral. The data transfer
characteristics are changed when the selected peripheral
changes according to the associated chip select register.
The PCS field in the SP_MR has no effect.
This option is only available when the SPI is programmed
in master mode.
Chip Selects
The chip select lines are driven by the SPI only if it is pro-
grammed in master mode. These lines are used to select
the destination peripheral. The PCSDEC field in SP_MR
(Mode Register) selects one to four peripherals (PCSDEC
= 0) or up to 15 peripherals (PCSDEC = 1).
If variable peripheral select is active, the chip select signals
are defined for each transfer in the PCS field in SP_TDR.
Chip select signals can thus be defined independently for
each transfer.
If fixed peripheral select is active, chip select signals are
defined for all transfers by the field PCS in SP_MR. If a
transfer with a new peripheral is necessary, the software
must wait until the current transfer is completed and then
change the value of PCS in SP_MR before writing new
data in SP_TDR.
The value on the NPCS pins at the end of each transfer
can be read in the SP_RDR (Receive Data Register).
By default, all NPCS signals are high (equal to one) before
and after each transfer.
Mode Fault Detection
A mode fault is detected when the SPI is programmed in
master mode and a low level is driven by an external mas-
ter on the NPCS0/NSS signal.
When a mode fault is detected, the MODF bit in the SP_SR
is set until the SP_SR is read and the SPI is disabled until
re-enabled by bit SPIEN in the SP_CR (Control Register).