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Initialization and Configuration
1111
SLAU723A – October 2017 – Revised October 2018
Copyright © 2017–2018, Texas Instruments Incorporated
External Peripheral Interface (EPI)
16.4.3.4 Speed of Transactions
The COUNT0 field in the EPIBAUD register must be configured to set the main transaction rate based on
what the slave device can support (including wiring considerations). The main control transitions are
normally 1/2 the baud rate (COUNT0 = 1) because the EPI block forces data versus control to change on
alternating clocks. When using dual chip selects, each chip select can access the bus using differing baud
rates by setting the CSBAUD bit in the EPIHBnCFG2 register. In this case, the COUNT0 field controls the
CS0n transactions, and the COUNT1 field controls the CS1n transactions. When using quad chip select
mode, the COUNT0 bit field of the EPIBAUD2 register controls the baud rate of CS2n and the COUNT1
bit field is programmed to control the baud rate of CS3n.
Additionally, the Host-Bus mode provides read and write wait states for the data portion to support
different classes of device. These wait states stretch the data period (hold the rising edge of data strobe)
and may be used in all four sub-modes. The wait states are set using the WRWS and RDWS bits in the
EPI Host-Bus n Configuration (EPIHBnCFGn) register. The WRWS and RDWS bits are enhanced with
more precision by WRWSM and RDWSM bits in the EPIHBnTIMEn registers. Note none of the wait state
configuration bits can be set concurrently with the BURST bit in the same EPIHBnCFGn register. See
for programming information.
Table 16-10. Data Phase Wait State Programming
RDWS or WRWS Encoding in
EPIHBnCFGn Register
RDWSM or WRWSM Encoding
in EPIHBnTIMEn Registers
Data Phase Wait States
0x0
1
1 EPI clock cycle
0x0
0
2 EPI clock cycles
0x1
1
3 EPI clock cycles
0x1
0
4 EPI clock cycles
0x2
1
5 EPI clock cycles
0x2
0
6 EPI clock cycles
0x3
1
7 EPI clock cycles
0x3
0
8 EPI clock cycles
The CAPWIDTH bit in EPIHBnTIMEn registers controls the delay between Host-Bus transfers. When the
CSBAUD bit is set and multiple chip selects have been configured in the EPIHBnCFG2 registers, delay
takes an additional clock cycle to adjust the clock rate of different chip selects.
Word read and write transactions can be enhanced through the enabling of the BURST bit in the
EPIHB16CFGn registers.
16.4.3.5 Sub-Modes of Host Bus 8 and 16
The EPI controller supports four variants of the host-bus model using 8 or 16 bits of data in all four cases.
The four sub-modes are selected using the MODE bits in the EPIHBnCFG register, and are:
1. Address and data are muxed. This scheme is used by many 8051 devices, some Microchip PIC parts,
and some ATmega parts. When used for standard SRAMs, a latch must be used between the
microcontroller and the SRAM. This sub-mode is provided for compatibility with existing devices that
support data transfers without a latch (that is, CPLDs). In general, the de-muxed sub-mode should
normally be used. The ALE configuration should be used in this mode, as all Host-Bus accesses have
an address phase followed by a data phase. The ALE indicates to an external latch to capture the
address then hold until the data phase. The ALE configuration is controlled by configuring the CSCFG
and CSCFGEXT field to be 0x0 in the EPIHBnCFG2 register. The ALE can be enhanced to access two
or four external devices with four separate CSn signals. By configuring the CSCFG field to be 0x3 and
the CSCFGEXT bit to be 0 in the EPIHBnCFG2 register, EPI0S30 functions as ALE, EPI0S27
functions as CS1n, and EPI0S26 functions as CS0n. When the CSCFG field is set to 0x0 and the
CSCFGEXT bit is set to 1 in the EPIHBnCFG2 register, EPI0S30 functions as ALE, EPIOS33 functions
as CS3n, EPIOS34 functions as CS2n, EPI0S27 functions as CS1n, and EPI0S26 functions as CS0n.
The CSn is best used for Host-Bus unmuxed mode, in which EPI address and data pins are separate.
The CSn indicates when the address and data phases of a read or write access are occurring.