Input / Output
The AS2650v2 contains a wide variety of IO and peripheral capabilities. These include GPIO, UART, SPI, timers and PWM generators.
Accessing IO and peripherals
On-die IO and peripherals are not memory-mapped. Instead, a set of special CPU instructions, wrte and rede are used. These instructions always have an argument of an 8-bit address. This address is used to select a register inside a IO or peripheral to read or write. The rest of this chapter describes register locations in terms of this IO address.
The IO address is internally decoded by splitting it into a device and register address. The most-significant two bits of the address are used to select one of four macros on the die, while the remaining 6 bits are used to address a particular register within that macro.
Address |
Macro |
|---|---|
|
GPIO Ports |
|
Timers/Counters |
|
Serial Ports |
|
SID - Sound Interface Device |
GPIO ports and alternate functions
Most IO functionality of the AS2650v2 is exposed through its two 8-bit GPIO ports PORTA and PORTB. By default, both ports act as bi-directional input/output ports who’s direction and state can be set by the processor. However, each pin can also be individually set to expose the functionality of another IO peripheral, as described in the following tables.
Port # |
Alt. pin name |
Direction |
Alt. function |
|---|---|---|---|
|
|
I |
Lowest-priority external interrupt |
|
|
O |
UART data out |
|
|
I |
UART data in |
|
|
O |
Timer Output of Timer/Counter 0 |
|
|
O |
Timer Output of Timer/Counter 1 |
|
|
O |
Output of PWM channel 0 |
|
|
O |
Output of PWM channel 1 |
|
|
I |
Highest-priority external interrupt |
Port # |
Alt. pin name |
Direction |
Alt. function |
|---|---|---|---|
|
|
I |
Priority 6 interrupt |
|
|
O |
Output of PWM channel 2 |
|
|
I |
External clock input for Timer/Counter 0 |
|
|
I |
External clock input for Timer/Counter 1 |
|
|
O |
SID DAC data #1 |
|
|
O |
SID DAC data #0 |
|
|
O |
SID DAC Latch Enable |
|
|
O |
SID DAC Serial Clock |
Register descriptions
DDRA/DDRB - Data Direction Register A/B
These registers set the direction of each individual port in PORTA and PORTB. A one equals an output port while a zero equals an input port.
Address: 0x00
Address: 0x01
PORTA/PORTB - Output data of PORTA/PORTB
The states of each port pin configured as an output is set in these registers.
Address: 0x02
Address: 0x03
PINA/PINB - Input data of PORTA/PORTB
Data read from each port pin configured as an input can be accessed by the processor by reading these registers. They may only be read.
Address: 0x05
Address: 0x06
SPA/SPB - Alternate function enable of PORTA/PORTB
These registers allow selection of the alternate function for each pin of PORTA or PORTB individually. For every bit that is a one in this register, the corresponding pin will cease to respond to configuration in DDRx and PORTx and will instead be connected to an on-die peripheral device. Both pin direction and data will be controlled by the peripheral.
Address: 0x04
Address: 0x08
IRQC - Interrupt Request Clear (for external interrupts)
Address: 0x07
Writing a one into any of the ICx bits in this register will clear that external interrupt request latch, removing the pending interrupt.
Additionally, this register can be read, in which case the bits reflect which interrupts are currently pending.
Interrupts
Three of the GPIO pins have an alternate function of an interrupt request. These are only active if the alternate function is active. However, pending interrupts will not be cleared by disabling the alternate function on these pins. The interrupts are edge-sensitive going low to high.
Timers/Counters
The AS2650v2 contains a set of three 16-bit up-counters capable of counting up either by a fixed rate driven by the processor clock and a prescaler (timer mode) or by an external clock input (counter mode).
Each timer/counter consists of a 16-bit count register, a 16-bit prescaler and a 16-bit “top” register. The prescaler acts as a clock divider for either the internal or external clock which provides the rate at which the counter is incremented. The counter increments until it reaches the value set in the top register, at which point it resets to 0 and begins counting up again.
Timers/counters 0 and 1 may also be used as frequency generators on the TO0 and TO1 pins by toggling these pins on each time they reach top and reset.
These signals are actually always generated, they must simply be exposed by selecting the alternative function on PA3 or PA4.
Only timers/counters 0 and 1 may use an external clock input or generate an output signal. Timer/counter 2 can only be driven using the internal clock.
All timers/counters may generate interrupts when reaching top.
All timers/counters may be loaded and read by the processor. As neither operation is possible in one pass due to the processor only being able to transfer 8-bits of data at a time, the process for doing so is as follows:
For reading a timer/counter’s value, it must be captured first by writing the Timer Capture Register. The captured value can then be read.
A timer/counter’s value can be loaded by writing into its capture data register LSB first. The first byte written will be buffered internally until the MSB is written. Only then are the LSB and MSB combined and copied into the counter. The 16-bit prescaler and top registers behave similarly, with the LSB having to be written first before the MSB, as they use the same buffering mechanism to ensure a complete 16-bit value is written.
Register descriptions
TS - Timer Setings
Address: 0x40
T0IE, T1IE and T2IE are the interrupt enables for timers/counters 0, 1 and 2 respectively. If set to a one and that timer reaches top, an interrupt is generated.
The priorities for these interrupts are:
Timer/Counter 0: IRQ1
Timer/Counter 1: IRQ2
Timer/Counter 2: IRQ5
IC1, IC2, IC5 are the interrupt clears for the timer/counter interrupts. These bits must be all zero for the setting bits to be changed. If any of these bits are not zero during a write operation, only the interrupt clear(s) will occur. No settings will be changed, making the remaining 5 bits don’t-care.
T0EXT, T1EXT are the external clock input enables for timers/counters 0 and 1. Setting either bit will put that timer/counter into “counter” mode, switching its clock source from the processor clock to an external clock input. The alternative function selects on PORTB must also be set to expose these clock inputs. The prescaler still applies to the external clock inputs.
Note: due to a hardware bug, attempts to read this register will return invalid data. Do not read this register.
TCR - Timer Capture Register
Timer/counter values may be captured by writing into this register, a which point every timer/counter’s current value will be copied into its corresponding Capture Data Register. The value written is discarded. The action of attempting to write to this IO address is what triggers the capture.
Address: 0x41
TxPRE - Timer 0/1/2 Prescaler
Address: 0x42
Address: 0x43
Address: 0x44
Address: 0x45
Address: 0x46
Address: 0x47
The final timer/counter up-count rate will be clock / (TxPRE + 1).
TxTOP - Timer 0/1/2 Top
Address: 0x48
Address: 0x49
Address: 0x4A
Address: 0x4B
Address: 0x4C
Address: 0x4D
TxCD - Timer 0/1/2 Capture Data
Address: 0x4E
Address: 0x4F
Address: 0x50
Address: 0x51
Address: 0x52
Address: 0x53
PWM
There are a total of three PWM channels integrated on the AS2650v2 which are driven by an independent timer using the processor clock. Only the pulse-width is adjustable. The PWM signals are always generated. Using them only requires selecting them in the alternate function selection for PORTA/PORTB.
Register descriptions
PWx - Pulse Width 0/1/2
These registers inversely adjust the pulse width of each channel from 0 (longest) to 254 (shortest). Setting a value of 255 will cause the output to be always on.
Address: 0x54
Address: 0x55
Address: 0x56
Serial Ports
There are two kinds of serial ports present on the AS2650v2: a UART and a SPI master port. The SPI port is the only IO peripheral with dedicated pins, but the UART must be exposed by activating the relevant alternate functions on PORTA.
Both have independent prescalers, with the UART additionally able to trigger a CPU interrupt on receipt of a complete character.
Register descriptions
UIE - UART Interrupt Enable
Address: 0x80
Writing a one to this register enables the UART to trigger an interrupt whenever it has received a full character.
UDIV - UART Clock Divider
Address: 0x81
Address: 0x82
This register defines the amount by which the processor clock is divided to arrive at the UART bitclock. The UART bitclock will be equal to CPU clock / (UDIV + 1).
STAT - Serial Status
Address: 0x83
This is the combined status register for all serial ports.
UBUSY is set if the UART is busy sending a character and cannot accept a new character at this time.
SBUSY is set if the SPI port is busy exchanging a byte.
UHB is set if the UART has received a complete character which is ready to be read by processor. This bit is cleared once the processor reads UDR.
SDIV - SPI Clock Divider
Address: 0x84
This register defines the amount by which the processor clock is divided to arrive at the SPI serial clock. The SPI serial clock will be equal to CPU clock / (SDIV * 2 + 1).
UDR - UART Data Register
Address: 0x85
This register is used to transfer characters in and out of the UART. Writing this register immediately begins a UART transfer. UBUSY is set and the written character is sent out serially over TXD at the rate defined in UDIV.
If this register is read, the last character received by the UART on the RXD line is retreived. This also clears the UHB flag and any pending UART interrupt.
SDR - SPI Data Register
Address: 0x86
This register is used to transfer bytes in and out of the SPI port. Writing this register immediately begins a SPI full-duplex transfer. SBUSY is set and the written byte is sent out serially over SDO at the rate defined in SDIV. Simultaneously, a byte is received over SDI. Once the transfer is complete and SBUSY is cleared, the SDR will contain the received byte, which can now be read by the processor.